Indexed map function

[jcranch]

This fills in a gap: we currently have:

• fmap: no indices, no possibility of removal
• filterMap: no indices, may remove elements
• mapBy: indexed, may remove elements

This is the fourth combination: an indexed map with no possibility of removal.

The rationale:

• In cases where one plans to use it, it is clearly slightly more efficient, since we don't have to check for emptiness when reassembling the structure.
• It helps people who wish to implement a structured typeclass hierarchy like that in the witherable package.

[jcranch]

I should mention - I don't regard this request as necessarily in final form. I'm happy to write tests if desired, and I'm happy to change the nomenclature.