Add traversal methods

[chrisdotcode]

The Haskell version of Data.Map has methods in which you can traverse a Map, and return a modified version of that map.

Some of those include: map, mapWithKey, traverseWithKey, mapAccm, etc..

To the best of my knowledge, none of this is currently implemented.

[hdgarrood]

map we already have, via the Functor Map instance. For the others, it's pretty trivial if you go via List (Tuple k v), although of course these extra intermediate representations are unnecessary, so it might be nice to have versions in this library that don't do that.