Also, adding M in grammar to be shorthand for $m:_M (vargetter) op in match context.

It is coherent now with the SentenceContext::is_inside(int abs_pos)
- SentenceContext::is_inside(Position pos) is added, while
Position::is_inside(SentenceContext) is removed

I.E. empty sentence shouldn't be passed anyway and that is what
Operator class is checking for. But otherwise, at the level of
operator, it can deal with a sentence that has current pos outside
of boundaries.

Previously it would mandate agreement was met on all categories
in a tag with enough categories, if such tag was found.
Now it checks for agreement only on the number of categories that
actually match, including the case of zero categories matching
(the latter does meet the weak agreement).

Clearer and better performing version, also more likely to actually work as expected :)

Note: still not implemented internals of agreement

The design doc for llook/rlook was incorrect, swapping the semantics per the updated docs and swapping argument order for llook.

Values cleanup and doxy: remove some redundancies, make the Begin/End constants in Position an enum, change tag_ref to contents

The agreement stuff isn't implemented yet.

- since the default value for Position is Nowhere now.
Also, remove reduntant header and change stream to ostream in to_strings

The concept has been reworked a bit.