AST Cleanup. Fixes #47 and #41.

Two small cleanups. Hope you don't mind I did them together.
Honestly, I don't know what's a good answer here. It feels strange to talk about "anchor" and "assertions" in the parser at the same time. E.g. you call the `parseAnchor()` function, which then consumes the grammar about `assertions` :/

So here is my idea:
- keep the replacement of `Assertion` → `Anchor`
- add a note, that the naming is not consistent with the spec

How do you feel about this?