Add convenient syntax for working with F[Either[E, A]]

[road21]

We interested in few handy methods, that would allow to do basic things with F[Either[E, A]] with additional context Monad[F] without wrapping it in EitherT
possible list is

def orElse(f: => F[Either[E, A]]): F[Either[E, A]]
def catchAll(e: E => F[A]): F[A]
def absolve(implicit R: Raise[F, E]): F[A]
def flatMapR[B](f: A => F[B]): F[Either[E, B]]
def flatMapL[E](f: E => F[E1]): F[Either[E1, A]]
def doubleFlatMap[B](f: A => F[Either[E, B]]): F[Either[E, B]]