rust-status: refactor status into a struct
The code for dirstate/status has grown too large for comfort, this is the
first of three patches that try to improve maintainability.
In this patch, refactoring dirstate's status into a struct allows for slimming
down function signatures drastically, keeping the mental (and maintenance)
burden lower, since pretty much all of them shared a few common arguments.
This had the pleasant side-effect of simplifying lifetimes a little. This has
no observable impact on performance.
The next patch will add/improve documentation and refactor some types. I tried
to keep new code down to a minimum in this patch because it's already pretty
Differential Revision: https://phab.mercurial-scm.org/D8661