compiler: simplify interface lowering

This commit simplifies the IR a little bit: instead of calling
pseudo-functions runtime.interfaceImplements and
runtime.interfaceMethod, real declared functions are being called that
are then defined in the interface lowering pass. This should simplify
the interaction between various transformation passes. It also reduces
the number of lines of code, which is generally a good thing.

compiler/testdata/interface.go

@@ -49,6 +49,15 @@ func isStringer(itf interface{}) bool {
 	return ok
 }
 
+type fooInterface interface {
+	String() string
+	foo(int) byte
+}
+
+func callFooMethod(itf fooInterface) uint8 {
+	return itf.foo(3)
+}
+
 func callErrorMethod(itf error) string {
 	return itf.Error()
 }