Add asymmetric padding

[staticfloat]

This goes on top of #714, since NNlib now has support for asymmetrical padding this provides the Flux-level API for it.

[MikeInnes]

May as well fold into #714?

[staticfloat]

Sure, if you like, just close #714 and merge this one when FluxML/Tracker.jl#11 is merged.

[staticfloat]

@MikeInnes looks like we're good to go, merge at your leisure.

[MikeInnes]

Thanks a lot @staticfloat.

I suspect it'd be cleaner to store the Dims object in the layer, rather than storing the same settings and reconstructing it on the fly. But that's not important for now.

[staticfloat]

I suspect it'd be cleaner to store the Dims object in the layer,

I thought of that, but considering that we need to re-create it for every differently-sized x, it wasn't high on my priority list. It may save us a little bit of time, but not that much. :P

[MikeInnes]

Ah yes, I didn't realise you're actually including the size of the input. This is probably the right way to do it then.