`PartialFunction` Does Not Support `ConstructionBase.jl`

[JoshuaBillson]

As discussed in FluxML/Metalhead.jl#286, PartialFunction cannot be reconstructed from its fields. This means it won't work by default with ConstructionBase, which means it won't work by default with Functors.

I've opened PR #12 that adds support for ConstructionBase.jl by implementing a new constructor for PartialFunction.

Here's an example demonstrating the fix:

using PartialFunctions, Flux, Metalhead

m = ResNet(18)

m |> Flux.gpu # this will no longer throw an error

[archermarx]

Fixed by #12. Let me know if anything doesn't work.