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/lexer.l

@@ -53,7 +53,7 @@ static void adjustLoc(YYLTYPE * loc, const char * s, size_t len)
 
 static Expr * unescapeStr(SymbolTable & symbols, const char * s, size_t length)
 {
-    string t;
+    std::string t;
     t.reserve(length);
     char c;
     while ((c = *s++)) {