Implement rethrow

[Avasil]

Something like

final def rethrow[E1 >: E, B](implicit ev: A <:< Either[E1, B]): BIO[E1, B]

to go from BIO[E, Either[E, B]] to BIO[E, B]

Prior art:

• https://github.com/typelevel/cats/blob/master/core/src/main/scala/cats/MonadError.scala#L39 (rethrow)
• https://github.com/zio/zio/blob/master/core/shared/src/main/scala/zio/ZIO.scala#L77 (absolve)

[bszwej]

I'll work on that