.arcconfig

rust-status: improve documentation and readability

This patch is 2/3 in the series to improve the dirstate status code. It adds a
number of common type aliases to add more obvious semantics to function
signatures, improves/adds documentation where necessary and improves one or two
patterns to be more idiomatic.

Differential Revision: https://phab.mercurial-scm.org/D8662

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