A unified mapK

[kailuowang]

For many T[F[_], A] or T[F[_]], we can add a mapK that does the following

class T[F[_], A] {
  def mapK[F[_], G[_]]()(f: F ~> G): T[G, A]`
}

This is non-abstracted part of #1815
The following types can have this mapK method

• WriterT
• Kleisli
• OptionT
• EitherT
• IdT
• EitherK
• Tuple2K
• Free
• FreeT
• Func
• Nested
• OneAnd

@andyscott offered to work on this

[LukaJCB]

Fixed by #2000