Implement `bimap` which combines `left_map` and `right_map`

[nasadorian]

bimap is a function from the Bifunctor typeclass commonly seen implemented for the Either type.

This library already has map_left and map_right, so the implementation would simply be a combination of the two. Would the maintainers be interested in adding this combinator?

Example as an extension:

trait EitherExt {
  type L;
  type R;
  fn bimap<M, S, F: Fn(Self::L) -> M, G: Fn(Self::R) -> S>(
    self,
    f: F,
    g: G,
  ) -> Either<M, S>;
}

impl<L, R> EitherExt for Either<L, R> {
  type L = L;
  type R = R;
  fn bimap<M, S, F: Fn(Self::L) -> M, G: Fn(Self::R) -> S>(
    self,
    f: F,
    g: G,
  ) -> Either<M, S> {
    self.map_left(f).map_right(g)
  }
}

[cuviper]

I'd call it map_either, if only because my first thought at "bimap" was the data structure.

The idea has limitations compared to .map_left(f).map_right(g), because you're forcing those two closures to coexist -- e.g. they can't use the same &mut capture. This is why either has a corresponding either_with. I suppose we could do the same here.

I would implement it with a plain match though.

[nasadorian]

Thanks @cuviper for your response. Though bimap is canonical in FP, I don't mind calling it map_either or map_both to keep in the spirit of this library.

I can add a map_either_with variant so both closures can capture the same context. PR coming.