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