Trie functions

A quick update: tries are a fundamental data structure in Vesta, and used as the backing store to dictionaries. Since tries have many useful properties, it would be great to expose these properties to the "user". I just committed the partial-key? and keys forms to the core foundation. partial-key? returns #t if 100% of the string is within the dictionary, #f if none is and N (which is an integer) for a matching subset. keys on the other hand, does what it sounds like: it returns all keys within a dictionary structure. Now, on top of the set/bag library, I'll also include a simple semantic processing library with the default build.