Add `hmap`

[colehaus]

Useful for changing the type of the underlying collection. The 'h' in "hmap" is for higher-order.

I could also easily be convinced to use the name hoist: http://hackage.haskell.org/package/mmorph-1.1.2/docs/Control-Monad-Morph.html#v:hoist.

[JordanMartinez]

I think hoist is a better name than hmap, especially so that it doesn't get confused with hmap.

However, the real question is whether it's worth adding this to a core library.

[JordanMartinez]

Or put differently, does it pass the Fairbairn Threshold?