maybe, either: remove S.maybeToEither and S.eitherToMaybe

[davidchambers]

Before:

S.maybeToEither (x)

S.eitherToMaybe

After:

S.maybe (S.Left (x))
        (S.Right)

S.either (S.K (S.Nothing))
         (S.Just)

I appreciate the concern @Avaq expressed in #644 (comment):

My only concern is that in implementing the entire matrix of conversions between a, Maybe a, Either a b, and Either b a, we might be increasing API surface drastically for functions that can be expressed as simple compositions.

If others agree with this change, I would like to merge this pull request then rebase and merge #644. If you do not think these two functions should be removed, please say so. :)

/cc @futpib, @masaeedu

[davidchambers]

I find the new implementation just as clear as the existing one. either (K (Nothing)) (Just) is the natural transformation from either to maybe. :)

[masaeedu]

Things like this can probably go into a hypothetical sanctuary lens library, which would make it easier to convert back and forth and fill in cells in the "conversion matrix" via ad-hoc composition.

[Avaq]

I think it's fine to remove these functions, as long as #644 is merged to provide alternatives.