Add missing semi-/sub-/-F/-Tap versions of flatMap to monad transformers

Monad transformers lack some semi-/sub-/-F/-Tap versions of flatMap that free a user from manual lifting or mapping.

I find the following items reasonable for implementation as to make structures and transformers more consistent:

• OptionT:
  • add member methods flatTap, flatTapF, semiflatTap, subflatTap
• Either:
  • add extensions flatTap, leftFlatTap to EitherOps syntax class
• EitherT:
  • add member methods leftFlatMapF, leftSubflatMap
  • add member methods flatTap, flatTapF, semiflatTap, subflatTap
  • add member methods leftFlatTap, leftFlatTapF, leftSemiflatTap, leftSubflatTap
  • add member methods biSemiflatTap
• Ior:
  • add member methods leftFlatMap
  • add member methods flatTap, leftFlatTap
• IorT:
  • add member methods leftFlatMapF, leftSubflatMap
  • add member methods flatTap, flatTapF, semiflatTap, subflatTap
  • add member methods leftFlatTap, leftFlatTapF, leftSemiflatTap, leftSubflatTap
• IdT:
  • add member methods flatTap, flatTapF
• WriterT:
  • add member methods flatMapF, semiflatMap, subflatMap
  • add member methods flatTap, flatTapF, semiflatTap, subflatTap
• Kleisli:
  • add member methods flatTap, flatTapF
• IndexedStateT:
  • add member methods flatTap, flatTapF
• IndexedReaderWriterStateT:
  • add member methods flatTap, flatTapF

Open questions:

• flatTap automatically comes from FlatMap syntax. For some transformers it has a special implementation (e.g. for EitherT), for others it is standard (e.g. for OptionT). Should we add it to the latter for symmetry and consistency? (Added)
• EitherT has biSemiflatMap. Do Either/EitherT/Ior/IorT need biFlatMap with all the semi-/sub-/-F/-Tap versions?

Start by reviewing the existing flatMap, biSemiflatMap, and related semi-/sub-/-F/‑Tap APIs across Either, EitherT, Ior, and IorT. The listed implementation items are marked complete, so the remaining work is to determine whether biFlatMap and its variants are needed and define what completion means before changing the transformer APIs.