lefts / rights / partitionEithers / bimap methods

[SandroMaglione]

lefts, rights, partitionEithers

This PR adds the lefts, rights, and partitionEithers methods to Either.

As for #48, previously it was hard to filter all the Right or Left values from a List of Either.

lefts and rights allows to extract only Right and Left values from an Either (same as their respective Haskell methods lefts and rights).

partitionEithers extracts both Left and Right values and collects them in a Tuple2 (partitionEithers).

Also added extension methods to List.

final list = [
  right<String, int>(1),
  right<String, int>(2),
  left<String, int>('a'),
  left<String, int>('b'),
  right<String, int>(3),
];
final result = Either.rights(list);
expect(result, [1, 2, 3]); // <- All `Right` values

final list = [
  right<String, int>(1),
  right<String, int>(2),
  left<String, int>('a'),
  left<String, int>('b'),
  right<String, int>(3),
];
final result = Either.lefts(list);
expect(result, ['a', 'b']); // <- All `Left` values

bimap

The bimap method was present in TaskEither, but missing in Either, IOEither, and Tuple2. Not anymore! Also added mapLeft to IOEither.