rust-filepatterns: add a Rust implementation of pattern-related utils This change introduces Rust implementations of two functions related to pattern handling, all located in `match.py`: - `_regex` - `readpatternfile` These utils are useful in the long-term effort to improve `hg status`'s performance using Rust. Experimental work done by Valentin Gatien-Baron shows very promising improvements, but is too different from the current Mercurial core code structure to be used "as-is". This is the first - albeit very small - step towards the code revamp needed down the line. Two dependencies were added: `regex` and `lazy_static`. Both of them will be useful for a majority of the Rust code that will be written, are well known and maintained either by the Rust core team, or by very frequent contributors. Differential Revision: https://phab.mercurial-scm.org/D6271