It was looking for minimum value and Position::Nowhere is
the smallest you can get value-wise (min_int). So if there was
an empty Match in a vector, first_token for the vector would
always end up returning Position::Nowhere, which was wrong
(empty submatches should've been ignored and Position::Nowhere
returned only if there were no non-empty alternatives).

So it's clear it's not about MatchRule at all.

MatchVector, AnnotationMatch, TokenMatch are now MatchData
MatchData is value_type of Match which now has get_value/set_value
Match has also copy construction working properly now
Default value for Match holds MatchVector so adding $m:_M var in grammar works as intended.

First draft of Match value type with some tests. MatchVector does not yet properly implement first/last token, hence a test fails, and AnnotationMatch is pretty much a stub.

Test for relabel. They will fail since there's no way of writing incomplete tags. To consider and either fix or remove the feature.

Note: one of the tests fails on purpose so everyone can see what it looks like.

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.