f1ef994f12
Using parallel marking in GC speeds up evaluation a fair bit
```
Benchmark 1: ./lix-main/bin/nix search nixpkgs hello --no-eval-cache
Time (mean ± σ): 20.740 s ± 0.046 s [User: 17.583 s, System: 3.062 s]
Range (min … max): 20.678 s … 20.794 s 10 runs
Benchmark 2: ./lix-parallel-marking/bin/nix search nixpkgs hello --no-eval-cache
Time (mean ± σ): 15.037 s ± 0.080 s [User: 19.602 s, System: 3.125 s]
Range (min … max): 14.960 s … 15.227 s 10 runs
Summary
./lix-parallel-marking/bin/nix search nixpkgs hello --no-eval-cache ran
1.38 ± 0.01 times faster than ./lix-main/bin/nix search nixpkgs hello --no-eval-cache
```
Based-on: https://github.com/NixOS/nix/pull/13708
Co-authored-by: Eelco Dolstra <edolstra@gmail.com>
Change-Id: Ibc7625f21e0ee7c8ad66203eeb3aca5d83977731