rust: Update the memmap2 crate to version 0.4.0
This version:
* Optionally implements the `StableDeref` trait, removing the need
for a wrapper type doing that
* Adds support for mapping empty files, which can routinely happen
for dirstate-v2 data files. This was a cause of some failing tests
when running with `run-tests.py --rhg --extra-config-opt rhg.status=1`
Differential Revision: https://phab.mercurial-scm.org/D11446
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