Generalise mapBoth to Bifunctor

[chshersh]

Currently we have the following mapBoth function in the Relude.Extra.Tuple module:

relude/src/Relude/Extra/Tuple.hs

Lines 64 to 65 in 31ad834

	mapBoth :: (a -> b) -> (a, a) -> (b, b)
	mapBoth f (a1, a2) = (f a1, f a2)

I propose to generalise the type signature of this function so it can work with any Bifunctor:

mapBoth :: Bifunctor f => (a -> b) -> f a a -> f b b

For this we need:

• Move mapBoth to Relude.Extra.Bifunctor
• Generalise and implement
• Add more examples to the documentation

[chshersh]

Instead of moving, let's introduce bimapBoth function and deprecate mapBoth first.

[josephcsible]

Should we do traverseBoth -> bitraverseBoth and generalize/move it too?

[chshersh]

@josephcsible I'm not sure about bitraverseBoth. I would like to keep relude minimal and not introduce rarely used combinators. I needed mapBoth enough times to be included in Extra modules but I never needed bitraverseBoth

[josephcsible]

We already have traverseBoth though, so we'd just be generalizing it, not adding a new one.

[chshersh]

@josephcsible This involves solving several awkward problems:

1. Deprecation cycle for traverseBoth should be done as well.
2. bitraverse is for Bitraversable while the module is called Bifunctor. It feels awkward to put Bitraversable function into a module called Bifunctor. And introduce module with the name Bitraversable looks like a heavy solution for generalising one rarely used function. I would love to have fewer modules, not more 🙂

[astynax]

I can take this one, but what I'll need to do specifically? :)

Instead of moving, let's introduce bimapBoth function and deprecate mapBoth first.

Just this part?

[chshersh]

@astynax Yep, only this part. Specifically:

1. Implement bimapBoth in the Relude.Extra.Bifunctor module
2. Add {-# DEPRECATE #-} pragma to mapBoth
3. Move documentation from mapBoth to bimapBoth and add couple extra usage examples using doctest (they will be very helpful!)