* Export `Numeric.Backprop.Internal.InpRef`

* Actually export internal constructors and utilities in `Numeric.Bac…

…kprop.Internal`.

* Export `Numeric.Backprop.Internal`.

* vinyl-0.14 compatibility (@msakai), which can be disabled via cabal…

… flag

*   GHC 9.0+ compatibility (@msakai)

Thanks to all generous contributors and commenters!

Version 0.2.6.4

*   Compatibility with ghc-8.10.1 (@tonyday567)

Version 0.2.6.3

*   Add `Backprop` instances for the various *vinyl* types.
*   Rewrite many `Backprop` instances over newtypes to coerce instead of go
    through Generics

* Fix a numerical bug that would occur when an input is used directly…

… as the

    result of a computation. (For example, `gradBP id` or `gradBP2 const`).
*   Some internal changes to strictness which offers some performance boosts in
    computation of gradients.

Version 0.2.6.1

*   Removed redundant constraints from `Floating` instance of `Op`.
*   Fixed lower bound for *vinyl* dependency.

Version 0.2.6.0

*   Dropped `Expr` instance of `Backprop`. I don't think anyone was actually
    using this.  If you need this, please use `Numeric.Backprop.Num` instead!
*   Removed *Rec* re-exports.
*   Compatibility with *vinyl-0.9*, using the *Data.Vinyl.Recursive* interface.
    This requires some minor reshuffling of constraints but should not affect
    any monomorphic usage.

Version 0.2.5.0

*   Since *type-combinators* has been unmaintained for over two years, and is
    no longer compatible with modern GHC, the library internals was rewritten
    to be built on the type-level combinators in the *vinyl* library instead.
    The main external API change is basically `Every` is replaced with
    `AllConstrained`, and `Known Length` is replaced with `RecApplicative`.

    To most users, this should make no difference API-wise.  The only users
    affected should be those using the "-N" family of functions (`backpropN`),
    who have to pass in heterogeneous lists.  Heterogeneous lists now must be
    passed in using *vinyl* syntax and operators instead of the previous
    *type-combinators* interface.
*   `bpOp` added, to allow for non-rank-N storage of backpropagatable
    functions in containers without impredicative types.
*   Benchmarks use *microlens* and *microlens-th* instead of *lens*.