New feature: For-comprehensions for Eithers and Validations

[adamkopec]

Allows for Either and Validation usage in for-comprehensions, eg:

Either<Object, Integer> result = For(
                Either.right(1),
                Either.right(2),
                Either.right(3)
            ).yield((i1, i2, i3) -> i1 + i2 + i3);
            assertThat(result.get()).isEqualTo(6);

[pivovarit]

This was very much lacking - thanks! I will give it a closer look and, quite likely, merge as is!