Skeleton: simplified path structure.

Skeleton: simplified path structure.

Just store pointers to skeleton nodes (instead of bookkeeping arcs,
contexts and rules in each path). All the necessary information can
be easily retrieved from nodes when path is baing dumped to file.

Three tests in '--skeleton' mode have been broken by this commit.
Actually, these are not breakages: these cases reveal incorrect
re2c-generated code. The change is due to the fact that skeleton
now doesn't simulate contexts that go *after* the matched rule:

    ------o------o------> ... (fallback to rule)
        rule   context