type checking and error handling for builtins improved.

object/int.go

@@ -6,6 +6,10 @@ type Integer struct {
 	Value int64
 }
 
+func (i *Integer) Bool() bool {
+	return i.Value != 0
+}
+
 func (i *Integer) Type() ObjectType {
 	return INTEGER_OBJ
 }