further improvements

internal/object/builtin.go

@@ -7,18 +7,18 @@ type Builtin struct {
 	Fn   BuiltinFunction
 }
 
-func (b *Builtin) Bool() bool {
+func (b Builtin) Bool() bool {
 	return true
 }
 
-func (b *Builtin) Type() Type {
+func (b Builtin) Type() Type {
 	return BuiltinType
 }
 
-func (b *Builtin) Inspect() string {
+func (b Builtin) Inspect() string {
 	return fmt.Sprintf("<built-in function %s>", b.Name)
 }
 
-func (b *Builtin) String() string {
+func (b Builtin) String() string {
 	return b.Inspect()
 }