rust: introduce `dirstate-tree` cargo feature
This feature gates (at compile-time) the use of the newly-added dirstate tree.
The motivation for this is that the dirstate tree is currently *very* slow;
replacing the current hashmap-based dirstate is not a viable solution in terms
of performance... and why would you be using the Rust implementation if not
for performance?
The feature will also help reviewers better understand the differences that
will slowly appear as the dirstate tree gets better.
Differential Revision: https://phab.mercurial-scm.org/D9132
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