Add modifyAscList, etc., to apply a potentially infinite list of modifications

Added `modifyAscList` and `modifyDecList`, and strict variants `modifyAscList'` and `modifyDecList'`, which take a list of indices paired with modify functions in strictly monotonic order and apply each modification to the trie. The monotonicity allows the list to be processed lazily as elements are requested, unlike repeated applications of `modify`. Also exports `mirror`, a utility function on tries which I used to implement `modifyDecList` in terms of `modifyAscList` and which may have utility in other contexts.

Prompted by this StackOverflow answer, which demonstrated an application for infinite lists of modifications which cannot be addressed with just `modify`: http://stackoverflow.com/revisions/25627918/6