Allow library external functions to be bound with using for

test/libsolidity/semanticTests/using/library_functions_inside_contract.sol

@@ -0,0 +1,31 @@
+library L {
+    function externalFunction(uint a) external pure returns (uint) { return a * 1; }
+    function publicFunction(uint b) public pure returns (uint) { return b * 2; }
+    function internalFunction(uint c) internal pure returns (uint) { return c * 3; }
+}
+
+contract C {
+    using {L.externalFunction} for uint;
+    using {L.publicFunction} for uint;
+    using {L.internalFunction} for uint;
+
+    function f() public pure returns (uint) {
+        uint x = 1;
+        return x.externalFunction();
+    }
+
+    function g() public pure returns (uint) {
+        uint x = 1;
+        return x.publicFunction();
+    }
+
+    function h() public pure returns (uint) {
+        uint x = 1;
+        return x.internalFunction();
+    }
+}
+// ----
+// library: L
+// f() -> 1
+// g() -> 2
+// h() -> 3