rust-dirstate: introduce intermediate struct for dirstate-v2 data This is passed often as a long tuple that is not easy to know the form of, so we refactor everything in this struct. This also renames `wdir_tracked` to follow the Python `wc_tracked`, even though the on-disk format uses `WDIR_TRACKED`. I think a single naming scheme is better, but we can't easily break the Python impl now because of extensions, so this is low-effort enough and facilitates grepping. Differential Revision: https://phab.mercurial-scm.org/D12498