Treat matching on the boundaries as an assertion

What the headline says ;) 

Also, this PR changes the `sub` property to `name` on the `type: assertion`. This is to make it more consistent with the `type:escaped` entry, which also has a `name` property.

I am not too much satisfied with the name `not-boundary`. Any better ideas are welcome :)
Might also want to test `\B` in a `CharacterClass` (it should error).