Skeleton: don't keep expanded range representation in nodes.

Skeleton: don't keep expanded range representation in nodes.

For the most part character range is represented as a pair of bounds.
When generating data for skeleton programs we need to pick some
256 distinct characters from each range; only these characters will
participate in data generation.

Before this commit range expansion happened at the time of skeleton
construction. Now it is delayed until the time of dumping skeleton
paths to file. This means that expansion of particular range is done
multiple times (every time the corresponding skeleton multiarc is
dumped to file). However, data generation only happens with '--skeleton'
option, while skeleton construction is unavoidable.