add Core.Some/None support for conversion to Either with Maybe

[jltrem]

currently given a function with return type Either<A, Maybe<B>> these returns work:

• return new Maybe<B>(value);
• return Maybe<B>.None;
• return left ? a : new Maybe<B>(value);
• return left ? a : Maybe<Country>.None;

I'd like to have the ability to:

• return Some(value);
• return None;
• return left ? a : Some(value);
• return left ? a : None;