v0.18.0 big rules

This is the first release under the new name, Lisien.

This release adds an option to mark any rule as "big," which enables an optimization to make a lot of changes to the world all at once.

An optimization trick I expect to be common is to combine all the rules that run every turn into one, changing very little about the rule code, and set that rule's "big" property to True.

There are new graphics in this release, thanks to Kenney's All-in-1 Game Asset Bundle.