add a hash value to Typeofwrapper objects

[vtjnash]

We probably should not do this, in full correctness, but the performance gain is too great to ignore. This is a very rare API mistake, but it occurred on a very popular constructor, making this very costly to construct a Broadcasted.

$ git grep 'struct.*<:Union{' base
base/broadcast.jl:struct Broadcasted{Style<:Union{Nothing,BroadcastStyle}, Axes, F, Args<:Tuple} <: Base.AbstractBroadcasted
base/compiler/ssair/ir.jl:struct InsertBefore{T<:Union{IRCode, IncrementalCompact}} <: Inserter
base/sort.jl:struct ScratchQuickSort{L<:Union{Integer,Missing}, H<:Union{Integer,Missing}, T<:Algorithm} <: Algorithm

[KristofferC]

but the performance gain is too great to ignore

A natural follow up question is what the actual numbers are.

[LilithHafner]

ScratchQuickSort is internal so we could make it struct ScratchQuickSort{L, H, T<:Algorithm} <: Algorithm if that would help with compile times.

[vtjnash]

I think it is okay. We should try to make this work okay and correctly.