v0.21.0 bookmarks

This release adds bookmarks, which let you assign names to points in time. Then you can travel between them more conveniently. It was always possible to store the (branch, turn, tick) yourself, but the new way is closer to what you'd expect if you've worked with save files, as in every game. Call `engine.bookmark('name')` once to create a bookmark, then call it again with the same name to travel back to that bookmark. You can also look up saved bookmarks as `engine.bookmark['name']`.

Currently, the Elide frontend doesn't have a good way for you to use the bookmarks. I'll add that next.

Databases from earlier Lisien versions are incompatible with this one, due to changes in the schema.