use more string_view in utils

there's a couple places that can be easily converted from using strings to using
string_views instead. gives a slight (~1%) boost to system eval.

 # before

  nix eval --raw --impure --expr 'with import <nixpkgs/nixos> {}; system'
    Time (mean ± σ):      2.946 s ±  0.026 s    [User: 2.655 s, System: 0.209 s]
    Range (min … max):    2.905 s …  2.995 s    20 runs

 # after

    Time (mean ± σ):      2.928 s ±  0.024 s    [User: 2.638 s, System: 0.211 s]
    Range (min … max):    2.893 s …  2.970 s    20 runs

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