Rename *WithIndex to Ix*

[well-typed]

Rename *WithIndex to Ix*

[arybczak]

Having internal classes with Ix prefix sounds fine to me, public ones can stay as-is so that people used to lens naming aren't confused.

[adamgundry]

I'm not sure about this. Morally the classes are the same as the ones in lens, aren't they? (Although it seems lens includes optics in the class dictionaries, presumably for performance reasons.) So in some sense our versions are really replacements for the lens ones and using the same names might make their relationship clearer.

[phadej]

A compromise is to have FunctorWithIndex, FoldableWithIndex, TraversableWithIndex, but name profunctors with Ix prefix.

Or then all *WithIndex (we have IxProfunctor now).

[arybczak]

Resolved via #49.