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/attr-path.hh

@@ -7,7 +7,7 @@
 
 namespace nix {
 
-Value* findAlongAttrPath(EvalState& state, const string& attrPath,
+Value* findAlongAttrPath(EvalState& state, const std::string& attrPath,
                          Bindings& autoArgs, Value& vIn);
 
 }