Add unzip instance for Getter

[swsnr]

Closes GH-455

@julien-truffaut While I was at it :)

[julien-truffaut]

if you get the time, could you also add unzip to the Getter class, the signature should probably look like this:

import scalaz.Leibniz.===
def unzip[B, C](implicit ev: (B, C) === A): (Getter[S, B], Getter[S, C])

but we should check that type inference works with something like:

getterA.zip(getterB).unzip

see also https://github.com/julien-truffaut/Monocle/blob/master/core/shared/src/main/scala/monocle/Prism.scala#L246

[swsnr]

I added the instance method to Getter, but it doesn't look like type inference can cope with it. The compiler won't find the implicit evidence for === unless you explicitly give type parameters to unzip.

[swsnr]

I had to give the type parameters to .unzip explicitly to make the compiler find the implicit evidence for ===. Without explicit type parameters it wouldn't compile.

[julien-truffaut]

arf ok, maybe we should revert back to your first commit if we can't get the type inference to work properly

[swsnr]

@julien-truffaut I dropped the .unzip instance method again.

[julien-truffaut]

@lunaryorn sorry for the back and force. Thank you very much 👍

[swsnr]

@julien-truffaut Never mind; I learned a bit about scalaz along the way for I never actually used it before.