maybe, either: remove S.toMaybe and S.toEither

[davidchambers]

Closes #465

These functions are often misused. I have seen code like this in the Gitter room on several occasions:

S.fromMaybe (y) (S.map (f) (S.toMaybe (x)))

I am removing these functions for several reasons:

• they necessitate the use of ? in type signatures;
• they produce expressions that are less clear about failure conditions than the equivalent ternary expressions or reject–Just compositions; and
• S.toMaybe reinforces the common misunderstanding that Maybe a exists solely as a replacement for Nullable a.