Remove unused boost include and split out std-hash.hh

Splitting it out immediately answers questions like [this],
without increasing the number of compilation units.

I did consider using boost::hash_combine instead, but it doesn't seem
to be quite as capable, accepting only two arguments.

[this]: https://github.com/NixOS/nix/pull/11113#discussion_r1679991573

src/libutil/meson.build

@@ -216,6 +216,7 @@ headers = [config_h] + files(
   'source-accessor.hh',
   'source-path.hh',
   'split.hh',
+  'std-hash.hh',
   'strings.hh',
   'strings-inline.hh',
   'suggestions.hh',