clean up parser.h

• variable look ahead can probably be removed.
• documentation
• fix ordering problem (probably by not returning anything:

  cont.token(open);
  return cont.process(open.someProp ? x : y, [](...) { ... });

• consider making all the function not static but regular, so Data does not need to be passed along (careful with the scope rule)
• consider early abort (probably should make a macro for this): if (!cont.accept(...)) return;