It was returning Position based off absolute position, which
was no good for operators like orth[first(M)] working in changing
sentence contexts.

This reflects the fact that positions stored in matches are
absolute positions, while Position type is used in the
language as position relative to current position in the
sentence context. So mixing the two was confusing at the very least.
This also actually fits the most recent version of spec,
which no longer mentions Position, either.

It didn't like passing reference to something that could be treated as a value.

Variables is noncopyable but default copy constructor was apparently working in VS.

Each subsequent rule was reusing variables object, which wasn't good.
Parsing of sequences had to be changed not to end with semi,
and it wasn't actually passing on lexicons in the first place.

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).