Closes #25.

This adds the following methods to fastmap:

• map(), map_with_key()
• filter(), filter_with_key(), filter_key()
• modify(), modify_with_key(): like map methods, but modifies in place.
• walk(), walk_with_key(): like map methods, but doesn't collect return values.

I'd also like to add methods to filter in place, but need a good name for it.