Drop 'either' dependency

[glguy]

The new 'either' package dependency drags in transitive dependencies on "transformers-base" "MonadRandom" "monad-control" into upstream packages like "lens".

This instance might not be worth the dependency cost.

[ocharles]

Why is that a problem? It's a useful instance for us :(

[ekmett]

A concern I have about it is that the instance isn't actually "free". One option might be to invert the dependency and make either depend on free.

Most of the users of errors bring in free anyways as the former is Gabriel's baby.

Given the existence of ExceptT, now EitherT is more of a fringe type.

[ekmett]

Moved the instance to either. Now the complaining can start anew there. ;)