speed-up compile-config

[grst]

For large projects, especially when on a slow-ish file system, compile-config can take seconds or even 10s of seconds.
We should try to make this faster.

• benchmark. We should identify bottlenecks before investing anywhere
• caching. do not recompile files if nothing has changed
• faster directory recursion: rust's ignore with python bindings: https://borsattoz.github.io/ignore-python/ignore.html