Add a cache to speed up the parser

[fain182]

Class parsing takes up 90% of the time, so having a cache could radically change performance.

Following @micheleorselli 's suggestion, something similar to what php-cs-fixer does might be enough: you serialize the ClassDescription and save it to a file, associating it with the hash of the file's content.
When you need to parse, you first compute the hash and look it up in the cache — if it's there, you hydrate the ClassDescription and skip parsing entirely.

One thing to watch out for: with inheritance support, a dependency's cache could become stale even if the individual file hasn't changed...