compiler: process interop together with package

Closes #913.
Provide package info in the funcScope to check if function is defined
insided an interop package. As a good side-effect bytecode for builtins from `util`
is no longer emitted.

Related #941.

pkg/compiler/testdata/util/equals.go

@@ -0,0 +1,6 @@
+package util
+
+// Equals is a mirror of `Equals` builtin.
+func Equals(a, b interface{}) bool {
+	return true
+}