Trie comprehensions abstract the process of projecting one-or-more
tries against some patterns, binding the results to usable variables,
and iterating over the results.
Currently supports for-trie/fold, for-trie/list, for-trie/set, and
for-trie/patch, as well as the ability to easily create new trie
comprehension macros. Note that these comprehensions operate in the
style of `for*` rather than `for`.