Add rxmark op.
Fill out compilation of scan.

Not much use yet since the backtracking logic isn't in place.
Emit much of the backtrack handling logic.

Doesn't handle the capture stack yet (but we don't have anything that
captures yet either). The rest is there, modulo thinkos.
Fix a thinko in scan, so it actually progresses.

With this, we can do do things like:

    say(('abcdef' ~~ /:i <[de]>F/).Str())