Issue #4385: Make Leo usable for large repos

[Edward K. Ream]

Issue #4385 recommends a significant addition to Leo. I want to make Leo usable for studying large repos such as Rust's compiler.

I have considered many alternatives not mentioned in the issue itself. Here are the takeaways:

- Only @clean can possibly do the job. @auto would be way too slow.

- Leo can (must!) avoid the @clean update algorithm when the external file has not changed.

- For changed files, a new postpass can be used to split nodes as necessary when changes define new functions or methods.

All comments on this project are welcome here or in the issue itself.

Edward