Merge #4489

Merge #4489

4489: Memory allocation optimization r=matklad a=simonvandel

I did some profiling using DHAT, and this was what I could easily optimize without much knowledge of the codebase.

This speeds up analysis-stats on rust-analyser by ~4% on my local machine.

**Benchmark**
➜  rust-analyzer-base git:(master) hyperfine --min-runs=2 '/home/simon/Documents/rust-analyzer/target/release/rust-analyzer analysis-stats .' '/home/simon/Documents/rust-analyzer-base/target/release/rust-analyzer analysis-stats .'
Benchmark #1: /home/simon/Documents/rust-analyzer/target/release/rust-analyzer analysis-stats .
  Time (mean ± σ):     49.621 s ±  0.317 s    [User: 48.725 s, System: 0.792 s]
  Range (min … max):   49.397 s … 49.846 s    2 runs

Benchmark #2: /home/simon/Documents/rust-analyzer-base/target/release/rust-analyzer analysis-stats .
  Time (mean ± σ):     51.764 s ±  0.045 s    [User: 50.882 s, System: 0.756 s]
  Range (min … max):   51.733 s … 51.796 s    2 runs

Summary
  '/home/simon/Documents/rust-analyzer/target/release/rust-analyzer analysis-stats .' ran
    1.04 ± 0.01 times faster than '/home/simon/Documents/rust-analyzer-base/target/release/rust-analyzer analysis-stats .'

Co-authored-by: Simon Vandel Sillesen <simon.vandel@gmail.com>