Improve stats

[cleverca22]

this adds percentages to the bytes in the stats
values allocated: 119 (2856 bytes 45%)
and also adds json:

[clever@amd-nixos:~/apps/nix]$ NIX_SHOW_STATS=json NIX_SHOW_STATS_PATH=foo.json ./result/bin/nix-instantiate --eval -E '5*5'
warning: unknown setting 'build-extra-platforms'
25
evaluation statistics:

[clever@amd-nixos:~/apps/nix]$ jq .values.bytes < foo.json 
2856

[edolstra]

Looks good. Could you remove the old stats format? I don't think we need to have both, JSON output is strictly better.

[cleverca22]

previously, NIX_SHOW_STATS was changing the verbosity, so the stats could still be seen with -vvvvv
but with the new code, it only shows the json when NIX_SHOW_STATS is non-zero, and it will write to a file if NIX_SHOW_STATS and NIX_SHOW_STATS_PATH are both set

the logic may need to be improved a little more

[vcunat]

Cross-link: NixOS/nixpkgs#53858