Try `mimalloc` instead of `jemalloc`

[Dav1dde]

Jemalloc while still working very well is no longer being maintained (Postmortem, Github).

We should try Microsoft's mimalloc, most likely only after their 3.X branch gets out of beta.

There is already a mimalloc Rust crate. Refer to this issue for 3.X support.

[linear[bot]]

RELAY-145 Try `mimalloc` instead of `jemalloc`