rustfmt.toml

rust: Make OwningDirstateMap generic and move it into hg-core

This will enable using it in rhg too.

The `OwningDirstateMap::new_empty` constructor is generic and accepts a value
of any type that gives acces to a bytes buffer. That buffer must stay valid
as long as the value hasn’t been dropped, and must keep its memory address
even if the value is moved. The `StableDeref` marker trait encodes those
constraints. Previously no trait was needed because the value was always
of type `PyBytes` which we know satisfies those constraints.

The buffer type is ereased in the struct itself through boxing and
dynamic dispatch, in order to simplify other signatures that mention
`OwningDirstateMap`.

Differential Revision: https://phab.mercurial-scm.org/D11396

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