refactor(3p/nix/libexpr): Use std::string as qualified type

Replaces most uses of `string` with `std::string`.

This came up because I removed the "types.hh" import from
"symbol-table.hh", which percolated through a bunch of files where
`string` was suddenly no longer defined ... *sigh*

third_party/nix/src/libexpr/value-to-xml.cc

@@ -8,7 +8,8 @@
 
 namespace nix {
 
-static XMLAttrs singletonAttrs(const string& name, const string& value) {
+static XMLAttrs singletonAttrs(const std::string& name,
+                               const std::string& value) {
   XMLAttrs attrs;
   attrs[name] = value;
   return attrs;