Add support for cat

[americast]

@MikeInnes Adds generic cat to Flux. Fixes #142.
The tests have been provided separately for cat as it needs a parameter which is not tracked.

[MikeInnes]

Looks good!

You don't need a new gradtest function here, however. You can just do gradtest((a, b) -> cat(2, a, b), ...) etc, as other tests for similar functions do.

[americast]

Thanks, @MikeInnes! Fixed it.

[MikeInnes]

Ok so two comments on this:

(1) You can avoid array allocations (and type instability) by working with tuples for till_xs and xs_in_Δ. e.g. ntuple(i -> _, Val{ndims(Δ)}).
(2) Can you extend this to n arguments? Doesn't have to be done as part of this PR, unless it's easy to do.

[MikeInnes]

Oh, and here's a test case that fails: @test gradtest((a, b) -> cat(2, a, b), rand(3), rand(3)).

[MikeInnes]

Merged with #243 -- thanks 👍