Use an LRU cache

[hegemonic]

For efficiency, Catharsis caches all of its parse results. Right now it caches all parse results indefinitely, which is not appropriate for a long-running process. Catharsis should use an LRU cache instead.

[hegemonic]

I've never verified that caching the parse results is actually faster than re-parsing the type expression. So before I add an LRU cache, I need to run some quick benchmarks of the following:

• Current implementation with caching
• Current implementation with caching, but using sigmund to generate the cache key
• Current implementation with caching disabled