Stacked borrow violation reported by miri

[Voultapher]

Hi, I've recently been doing research on sort implementations and noticed that dmsort exhibits stacked borrow violations when sorting random inputs. It should be relatively easy to reproduce by sorting a random set of numbers using miri. If you want you can reuse my project to reproduce it, https://github.com/Voultapher/sort-research-rs change the the test_sort line in tests/main.rs to use sort_comp::unstable::rust_dmsort as test_sort; and run it with MIRIFLAGS='-Zmiri-permissive-provenance' cargo +nightly miri t --features=emilk_dmsort -- --exact random.

Update: The code passes the test suite with Tree Borrows, so whether or not that is a "real" issue remains to be seen.