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.