rearrange builtins

builtins/lower.go

@@ -0,0 +1,18 @@
+package builtins
+
+import "monkey/object"
+
+import "strings"
+
+// Lower ...
+func Lower(args ...object.Object) object.Object {
+	if len(args) != 1 {
+		return newError("wrong number of arguments. got=%d, want=1",
+			len(args))
+	}
+
+	if str, ok := args[0].(*object.String); ok {
+		return &object.String{Value: strings.ToLower(str.Value)}
+	}
+	return newError("expected `str` argument to `lower` got=%T", args[0])
+}