compiler: add func tests

This is basically just a golden test for the "switch" style of func
lowering. The next commit will make changes to this lowering, which will
be visible in the test output.

compiler/testdata/func.go

@@ -0,0 +1,12 @@
+package main
+
+func foo(callback func(int)) {
+	callback(3)
+}
+
+func bar() {
+	foo(someFunc)
+}
+
+func someFunc(int) {
+}