Customizable lazy fused broadcasting in pure Julia

This patch represents the combined efforts of four individuals, over 60
commits, and an iterated design over (at least) three pull requests that
spanned nearly an entire year (closes #22063, #23692, #25377 by superceding
them).

This introduces a pure Julia data structure that represents a fused broadcast
expression.  For example, the expression `2 .* (x .+ 1)` lowers to:

```julia
julia> Meta.@lower 2 .* (x .+ 1)
:($(Expr(:thunk, CodeInfo(:(begin
      Core.SSAValue(0) = (Base.getproperty)(Base.Broadcast, :materialize)
      Core.SSAValue(1) = (Base.getproperty)(Base.Broadcast, :make)
      Core.SSAValue(2) = (Base.getproperty)(Base.Broadcast, :make)
      Core.SSAValue(3) = (Core.SSAValue(2))(+, x, 1)
      Core.SSAValue(4) = (Core.SSAValue(1))(*, 2, Core.SSAValue(3))
      Core.SSAValue(5) = (Core.SSAValue(0))(Core.SSAValue(4))
      return Core.SSAValue(5)
  end)))))
```

Or, slightly more readably as:

```julia
using .Broadcast: materialize, make
materialize(make(*, 2, make(+, x, 1)))
```

The `Broadcast.make` function serves two purposes. Its primary purpose is to
construct the `Broadcast.Broadcasted` objects that hold onto the function, the
tuple of arguments (potentially including nested `Broadcasted` arguments), and
sometimes a set of `axes` to include knowledge of the outer shape. The
secondary purpose, however, is to allow an "out" for objects that _don't_ want
to participate in fusion. For example, if `x` is a range in the above `2 .* (x
.+ 1)` expression, it needn't allocate an array and operate elementwise — it
can just compute and return a new range. Thus custom structures are able to
specialize `Broadcast.make(f, args...)` just as they'd specialize on `f`
normally to return an immediate result.

`Broadcast.materialize` is identity for everything _except_ `Broadcasted`
objects for which it allocates an appropriate result and computes the
broadcast. It does two things: it `initialize`s the outermost `Broadcasted`
object to compute its axes and then `copy`s it.

Similarly, an in-place fused broadcast like `y .= 2 .* (x .+ 1)` uses the exact
same expression tree to compute the right-hand side of the expression as above,
and then uses `materialize!(y, make(*, 2, make(+, x, 1)))` to `instantiate` the
`Broadcasted` expression tree and then `copyto!` it into the given destination.

All-together, this forms a complete API for custom types to extend and
customize the behavior of broadcast (fixes #22060). It uses the existing
`BroadcastStyle`s throughout to simplify dispatch on many arguments:

* Custom types can opt-out of broadcast fusion by specializing
  `Broadcast.make(f, args...)` or `Broadcast.make(::BroadcastStyle, f, args...)`.

* The `Broadcasted` object computes and stores the type of the combined
  `BroadcastStyle` of its arguments as its first type parameter, allowing for
  easy dispatch and specialization.

* Custom Broadcast storage is still allocated via `broadcast_similar`, however
  instead of passing just a function as a first argument, the entire
  `Broadcasted` object is passed as a final argument. This potentially allows
  for much more runtime specialization dependent upon the exact expression
  given.

* Custom broadcast implmentations for a `CustomStyle` are defined by
  specializing `copy(bc::Broadcasted{CustomStyle})` or
  `copyto!(dest::AbstractArray, bc::Broadcasted{CustomStyle})`.

* Fallback broadcast specializations for a given output object of type `Dest`
  (for the `DefaultArrayStyle` or another such style that hasn't implemented
  assignments into such an object) are defined by specializing
  `copyto(dest::Dest, bc::Broadcasted{Nothing})`.

As it fully supports range broadcasting, this now deprecates `(1:5) + 2` to
`.+`, just as had been done for all `AbstractArray`s in general.

As a first-mover proof of concept, LinearAlgebra uses this new system to
improve broadcasting over structured arrays. Before, broadcasting over a
structured matrix would result in a sparse array. Now, broadcasting over a
structured matrix will _either_ return an appropriately structured matrix _or_
a dense array. This does incur a type instability (in the form of a
discriminated union) in some situations, but thanks to type-based introspection
of the `Broadcasted` wrapper commonly used functions can be special cased to be
type stable.  For example:

```julia
julia> f(d) = round.(Int, d)
f (generic function with 1 method)

julia> @inferred f(Diagonal(rand(3)))
3×3 Diagonal{Int64,Array{Int64,1}}:
 0  ⋅  ⋅
 ⋅  0  ⋅
 ⋅  ⋅  1

julia> @inferred Diagonal(rand(3)) .* 3
ERROR: return type Diagonal{Float64,Array{Float64,1}} does not match inferred return type Union{Array{Float64,2}, Diagonal{Float64,Array{Float64,1}}}
Stacktrace:
 [1] error(::String) at ./error.jl:33
 [2] top-level scope

julia> @inferred Diagonal(1:4) .+ Bidiagonal(rand(4), rand(3), 'U') .* Tridiagonal(1:3, 1:4, 1:3)
4×4 Tridiagonal{Float64,Array{Float64,1}}:
 1.30771  0.838589   ⋅          ⋅
 0.0      3.89109   0.0459757   ⋅
  ⋅       0.0       4.48033    2.51508
  ⋅        ⋅        0.0        6.23739
```

In addition to the issues referenced above, it fixes:

* Fixes #19313, #22053, #23445, and #24586: Literals are no longer treated
  specially in a fused broadcast; they're just arguments in a `Broadcasted`
  object like everything else.

* Fixes #21094: Since broadcasting is now represented by a pure Julia
  datastructure it can be created within `@generated` functions and serialized.

* Fixes #26097: The fallback destination-array specialization method of
  `copyto!` is specifically implemented as `Broadcasted{Nothing}` and will not
  be confused by `nothing` arguments.

* Fixes the broadcast-specific element of #25499: The default base broadcast
  implementation no longer depends upon `Base._return_type` to allocate its
  array (except in the empty or concretely-type cases). Note that the sparse
  implementation (#19595) is still dependent upon inference and is _not_ fixed.

* Fixes #25340: Functions are treated like normal values just like arguments
  and only evaluated once.

* Fixes #22255, and is performant with 12+ fused broadcasts. Okay, that one was
  fixed on master already, but this fixes it now, too.

* Fixes #25521.

* The performance of this patch has been thoroughly tested through its
  iterative development process in #25377. There remain [two classes of
  performance regressions](#25377) that Nanosoldier flagged.

* #25691: Propagation of constant literals sill lose their constant-ness upon
  going through the broadcast machinery. I believe quite a large number of
  functions would need to be marked as `@pure` to support this -- including
  functions that are intended to be specialized.

(For bookkeeping, this is the squashed version of the [teh-jn/lazydotfuse](JuliaLang/julia#25377)
branch as of a1d4e7ec9756ada74fb48f2c514615b9d981cf5c. Squashed and separated
out to make it easier to review and commit)

Co-authored-by: Tim Holy <tim.holy@gmail.com>
Co-authored-by: Jameson Nash <vtjnash@gmail.com>
Co-authored-by: Andrew Keller <ajkeller34@users.noreply.github.com>

base/statistics.jl

@@ -145,7 +145,7 @@ function centralize_sumabs2!(R::AbstractArray{S}, A::AbstractArray, means::Abstr
         return R
     end
     indsAt, indsRt = safe_tail(axes(A)), safe_tail(axes(R)) # handle d=1 manually
-    keep, Idefault = Broadcast.shapeindexer(indsAt, indsRt)
+    keep, Idefault = Broadcast.shapeindexer(indsRt)
     if reducedim1(R, A)
         i1 = first(indices1(R))
         @inbounds for IA in CartesianIndices(indsAt)