Add a 'call_indirect' builtin to emit arbitrary calls (might trap at runtime); Optimize 'for' loop compilation a bit

tests/compiler/class-overloading.ts

@@ -0,0 +1,12 @@
+class Foo {
+  baz(): void {}
+}
+class Bar extends Foo {
+  baz(): void {}
+}
+export function test(foo: Foo): void {
+  foo.baz();
+}
+// FIXME: this results in a call to Foo.baz instead of Bar.baz above.
+// ultimately, overloaded functions should implicitly become virtual.
+test(changetype<Bar>(0));