Avoid costly forwarding.
Test of incremental lexing.
Delay combinators force to combinators.
Rearrange.
HMRCombinator abstracts nullability.

We don’t want to use -parseForest for this, because then reductions are
evaluated by concatenation.
Everything which parses null computes nullability.
Use nullability instead of parseForest for concat.
Merge pull request #47 from robrix/nullability

Nullability
Merge remote-tracking branch 'origin/master' into lexer
Add a test which triggers a weird crash.
Rename _nullability to _nullable.