Added yet another intermediate representation (after AST, before NFA).

Added yet another intermediate representation (after AST, before NFA).

Rationale: AST must be immutable, because it is shared among different
regexps, rules, conditions and blocks. NFA is mutable, but inconvenient
for some transformations (adding fixed and default tags, rewriting parts
of regexp, etc.). Before this commit all these transformations were done
in one pass (during convertion of AST to NFA) which made the code rather
complex. In order to add POSIX captures we'll need to add even more
complexity (tracking captures, creating tags for them, maintaining tag
order, etc.).