Backup overwritten tags in fallback states.

Backup overwritten tags in fallback states.

We need to backup tags in fallback states: these tags may be
updated on some non-accepting path from fallback state, and if
the attempt to match longer rule fails, there's no way to restore
the overwritten tags.

Such tags may appear in self-overlapping rules, e.g.:
    (@p "ab")* {}

We create a special backup variable for each potentially overwritten
tag. Backup tags do not participate in deduplication.