Fold output expressions in a separate pass over the output.

Fold output expressions in a separate pass over the output.

re2c folds sequences of simple statements to complex tatements, e.g.:
    ++YYCURSOR;
    yych = *YYCURSOR;
is folded to:
    yych = *++YYCURSOR;

It is hard to fold expressions on the fly (at the time they are generated),
because one needs to forsee all the cases in advance. Some cases cannot be
anticipated (e.g. if substatements belong to different states, which is often
the case), so many statements remain unfolded.

This commit separates code generation and folding by delaying output of
skip, peek and backup primitives. Folding now is done in a separate pass
when all the substatements are already known.