Correct location calculations for non-terminals.

The prior code constructed the location of nonterminal maches out of
the the state of the stream after parsing.  This isn't right for a few
reasons:

    1.  It doesn't get starting location correctly.

    2.  It doesn't behave when the non-terminal production did not
        actually consume tokens for its parse.

This patch modifies the parsers to also pass along a
"last-consumed-token"; it, along with a few other changes, provides
the parsers enough information to accurately construct the locations,
even when no tokens have been consumed during the parse.  We
synthesize a sentinel last-consumed-token token to take location from
the head of the stream.