6946: Better fuzzy heuristics r=matklad a=SomeoneToIgnore
Continuation of the https://github.com/rust-analyzer/rust-analyzer/pull/6922, mainly created for a test.
Turns out our current completions tests were sorting the completions by label, I had to remove that to test the order properly and update this order in a bunch of tests (ergo the changes)
Co-authored-by: Kirill Bulatov <mail4score@gmail.com>