Support lookahead assertions. Given a lookahead expression and a contingent sequence, here's what has to happen:

Make a lookahead state at the engine level. It will essentially be a normal expression state of [lookahead, contingent] with a bit of special sauce in handling success and failure.

• When lookahead succeeds it must fail the lookahead expression, causing contingent to become the best alternative
• When lookahead fails the entire lookahead state must fail.