Contract level checker: Disallow free function redefinition and alias

shadowing another free function

Co-authored-by: chriseth <chris@ethereum.org>

test/libsolidity/syntaxTests/multiSource/alias_shadows_function.sol

@@ -0,0 +1,19 @@
+==== Source: s1.sol ====
+function f() pure returns (uint) { return 1337; }
+function g() pure returns (uint) { return 42; }
+==== Source: s2.sol ====
+import {f as g} from "s1.sol";
+==== Source: s3.sol ====
+// imports f()->1337 as g()
+import "s2.sol";
+// imports f()->1337 as f() and
+// g()->42 as g
+import "s1.sol";
+contract C {
+  function foo() public pure returns (uint) {
+    // calls f()->1337 / f()->1337
+    return f() / g();
+  }
+}
+// ----
+// DeclarationError 1686: (s1.sol:0-49): Function with same name and parameter types defined twice.