Rename id_manager to document_state.
Remove `document_root` member from `file_info`.

It isn't actually used any more.
Make member_action and friends available in actions.

A fairly clumsy mechansim for calling member functions in parsers.
Document the element flags and masks a bit better.

I'm considering how source_mode tags work for paragraphs, which touches
on the contextual block stuff, so I wanted to clarify it.

As always, contains some TODOs that might not ever be dealt with.
Use a slightly clearer name for the grammar's block mask.
Store source mode as tag rather than string.
Start 1.7