Merge #5244

5244: Add a command to compute memory usage statistics r=matklad a=jonas-schievink

This allows inspecting memory usage on a live rust-analyzer instance after it has been used interactively.

This will only work with `--features jemalloc`, so maybe it should print something more useful when that's not available? Right now it will just print 0 Bytes for every query.

Co-authored-by: Jonas Schievink <jonas.schievink@ferrous-systems.com>