*   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.