Javadoc
For reference to rule r, build
o-e->(r) o
where (r) is the start of rule r and the trailing o is not linked
to from rule ref state directly (it's done thru the transition(0)
RuleClosureTransition.
If the rule r is just a list of tokens, it's block will be just
a set on an edge o->o->o-set->o->o->o, could inline it rather than doing
the rule reference, but i'm not doing this yet as I'm not sure
it would help much in the ATN->DFA construction.
TODO add to codegen: collapse alt blks that are sets into single matchSet