Rename lens values; fix `-Wall` warnings.

The renaming made sense to me but hasn't been discussed at all. I guess I had a little bit of a fit.

I switched the meaning of the trailing `'` to parallel its use in `lens`, e.g. `Lens` vs. `Lens'`, where the trailing prime indicates the *less* general variant. For internal functions, the trailing E means that it takes an `Elem` value rather than using `Implicit`.