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

src/libutil/types.hh

@@ -22,6 +22,7 @@ typedef std::map<string, string> StringMap;
 /* Paths are just strings. */
 
 typedef string Path;
+typedef std::string_view PathView;
 typedef list<Path> Paths;
 typedef set<Path> PathSet;