Add HashMap.transform and HashMap.unorderedTransformA

[johnynek]

This adds two methods to HashMap:

1. transform which matches the standard library function on Maps.
2. unorderedTransformA which is the applicative generalization of transform appropriate for an unordered collection (requiring a CommutativeApplicative).

Then we can use unorderedTransformA to implement unorderedTraverse which does strictly less work since these two new methods never rehash and reconstruct the trie, it just reuses the exact same structure as the original trie.

[gemelen]

LGTM
(note that at the moment I'm not in a capacity to grasp everything behind the changeset)