WIP: RFC: cache parse results while loading files

This is my attempt at partial caching of the results of loading `.jl` files. It is functional, but since it does not detect file changes yet, it is a little unsafe right now.

On the julia machine, it reduced the load time of Tk from 4.3 seconds to 2.9 seconds. On my Mac, it reduced the time from ~10 s to ~6 s.