Add checking for declaration of reserved variable names (writer and self)

[PizzasBear]

Check for declaration of variables named writer or self when parsing a target, and error if found.

[djc]

Did you actually run into this? What's your use case, anyway?

[PizzasBear]

I didn't run into it, but I just thought it'd be useful for someone who does.
The name writer could, for example, refer to the writer of a blog, and properly erroring would be helpful then.

[djc]

Okay, I think think this complexity has diminishing returns so I'd rather avoid this.

[Kijewski]

IMO that's a good addition. I don't think people will try to use self as an argument name, but I can see people stumble over the name writer.

[djc]

Alright, let's do it.