Replace every instance of `Either<Some, Failure>` to more explicit type

[BriceCroix]

The Either type is not an explicit result on its underlying data. It would be easier to deal with a type such as Rust's Result<Type, ErrorType>.

This package provides such type, that matches a swift one (not a Swift developper, I cannot tell).

But it is also be possible to write a custom wrapper around dartz's Either.

[BriceCroix]

closed by #17