Moved nontrivial context handling from parser to NFA construction phase.

Moved nontrivial context handling from parser to NFA construction phase.

Parser should simply construct AST; all the complex reasoning about
contexts (fixed vs variable) should be delayed until NFA is being
constructed. This way AST can be immutable and it's very easy to share
parts of AST between different conditions, etc.

Removed rule ranks and rank counter: rules are now stored in NFA-local
array and addressed by index.