Fix VM memory allocation optimizations by reducing what we allocate on the heap

internal/builtins/len.go

@@ -15,7 +15,7 @@ func Len(args ...object.Object) object.Object {
 	}
 
 	if size, ok := args[0].(object.Sizeable); ok {
-		return &object.Integer{Value: int64(size.Len())}
+		return object.Integer{Value: int64(size.Len())}
 	}
 	return newError("TypeError: object of type '%s' has no len()", args[0].Type())
 }