Symbol index caching

Creating a symbol index of a well filled sys.path takes a long time. Caching would help, but raises questions.
- granularity: cache whole indices (serialization with metadata about index search path and indexed modules) or symbols of individual modules
- format: JSON is (somewhat) human readable, pickle is (probably, untested!) faster but unsafe (does it matter?)
- invalidation: go by size/mtime or content hash
- location: let the caller decide, or use a global location like `~/.cache/importmagic`