Changed '.keys' file format (generated with '--skeleton').

Changed '.keys' file format (generated with '--skeleton').

Store length of strings instead of pointers to string start and end.

Storing pointers requires us to remember total length of strings already
written to file by the time we want to write next string. This is very
inconvenient if we want to dump strings as we perform DFS on graph:
we'll have to track size all the time (path-cover-generator already does
that, but all-paths-generator doesn't).

This adds a new local variable to the generated code ('token'), which
is used to backup cursor position when enterind DFA.