refactor: move ConstructPatterns into compiler package

ConstructPatterns was never used except during compilation, so it
makes sense to have it closer where it is used. We could consider
making it even private function, as the pattern table construction
is quite specific to how compiler compiles and probably not that
reusable elsewhere.

vm/compiler/compiler.go

@@ -91,7 +91,7 @@ func (com *Compiler) Song(song *sointu.Song) (map[string]string, error) {
 	if err != nil {
 		return nil, fmt.Errorf(`could not encode patch: %v`, err)
 	}
-	patterns, sequences, err := vm.ConstructPatterns(song)
+	patterns, sequences, err := ConstructPatterns(song)
 	if err != nil {
 		return nil, fmt.Errorf(`could not encode song: %v`, err)
 	}