Extend using-for.

test/libsolidity/syntaxTests/using/using_free_functions.sol

@@ -0,0 +1,16 @@
+function id(uint x) pure returns (uint) {
+    return x;
+}
+
+function zero(uint) pure returns (uint) {
+    return 0;
+}
+using {id} for uint;
+contract C {
+    using {zero} for uint;
+
+    function g(uint z) pure external {
+        z.zero();
+        z.id();
+    }
+}