Concurrent search

[pythonesque]

Allow finding and matching results concurrently while editing (including searching the in-progress buffer). This should be especially useful with large files. The results may be slightly stale, but there are ways of "fixing up" most results.

Doing this in a nonblocking way requires consistent use of transactions and MVCC, so it is not an easy problem, but it is a solved one and it would be really useful in a text editor.