rust-matchers: use the `regex` crate Instead of falling back to Python when a code path with "ignore" functionality is reached and `Re2` is not installed, the default compilation (i.e. without the `with-re2` feature) will use the `regex` crate for all regular expressions business. As with the introduction of `Re2` in a previous series, this yields a big performance boost compared to the Python + C code in `status`, `diff`, `commit`, `update`, and maybe others. For now `Re2` looks to be faster at compiling the DFA (1.5ms vs 5ms for Netbeans' `.hgignore`) and a bit faster in actual use: (123ms vs 137ms for the parallel traversal of Netbeans' clean repo). I am in talks with the author of `regex` to see whether that performance difference is a bug, a "won't fix", or a tuning issue. The `regex` crate is already one of our dependencies and using this code does not require any additional work from the end-user than to use the Rust extensions. Differential Revision: https://phab.mercurial-scm.org/D8323