type checking and error handling for builtins improved.

object/builtin.go

@@ -7,12 +7,18 @@ type Builtin struct {
 	Fn   BuiltinFunction
 }
 
+func (b *Builtin) Bool() bool {
+	return true
+}
+
 func (b *Builtin) Type() ObjectType {
 	return BUILTIN_OBJ
 }
+
 func (b *Builtin) Inspect() string {
 	return fmt.Sprintf("<built-in function %s>", b.Name)
 }
+
 func (b *Builtin) String() string {
 	return b.Inspect()
 }