toLeopard: Sprites and the stage shouldn't share the same reserved name list

There's plenty overlap between them, but also certain properties that are only specially meaningful on one of the two!

Sharing the same list doesn't cause any behavior harm, but it's overly cautious and can result in not-nice code. This will be a particularly big deal when variables are stored on `this` rather than `this.vars` (leopard-js/leopard#196).
