Adding biSemiFlatMap to EitherT

[ericaovo]

Looking into the methods available on an EitherT instance I came across the need of having something like the following:

def biSemiflatMap[C, D](fa: A => F[C], fb: B => F[D]): EitherT[F, C, D] =
  eitherT.leftSemiflatMap(fa).semiflatMap(fb)

Here's an example:

import cats.data._, cats.implicits._, cats.Monad

val eitherT: EitherT[List, String, Int] = EitherT(List(Left("abc"), Right(123)))

implicit class EitherTOps[F[_]: Monad, A, B](val eitherT: EitherT[F, A, B]) {
  def biSemiflatMap[C, D](fa: A => F[C], fb: B => F[D]): EitherT[F, C, D] =
    eitherT.leftSemiflatMap(fa).semiflatMap(fb)
}

eitherT.biSemiflatMap(string => List(string.length), int => List(int.toFloat))
// res: cats.data.EitherT[List,Int,Float] = EitherT(List(Left(3), Right(123.0)))

Happy to open a PR if it sounds reasonable.