Request to simplify API wrt Right forms

[aztecrex]

all right... and ...rights

There is now a proliferation of right... and ...Right methods in
the class. This makes for a really wide interface. I want to consider
a simplification.

Note on #getRight(): that seems OK and should probably stay.

For everything else, what about a #swap() (or maybe a better name) method like so:

// for any...
final Either<L,R> e = ...;
// let...
final Either<R,L> ep = e.swap();

// then...
assert e.isLeft() != ep.isLeft();
assert e.isLeft() || e.getRight().equals(ep.getLeft());
assert !e.isLeft() || e.getLeft().equals(ep.getRight());

So, rather than, e.g.:

e.rightOrElse("other");
e.maybeRight();

You would use:

e.swap().orElse("other");
e.swap().maybe();

Here is something interesting:

Either<Integer, String> e = 
  Either.of(Something::thatMightThrow)
     .swap().map(RuntimeException::getMessage)
     .swap()
     .map(My::method)
     ...
     ...
     ; 

Without #swap(), Either would need a #rightMap(..) or similar to
achieve the above.

I am not fully this is a better interface model. Would like to
see comments from others before making a decision.

[aztecrex]

This has been implemented and release in 0.10.

Still not sure 'swap' is the right method name. Hoping to think of a better one before 1.0.