Warn about using X for * in expressions via "I didn't understand that" style parser message

I ended up making this two ways and am submitting both for perusal.

This one gives feedback via the "I didn't understand that" parser message interface.  It's uglier than the other one, e.g. it needs the xUsedAsMultOp test defined twice because there doesn't seem to be a good place to put a function that the validator and setup code can both talk to.  However, I'm still putting it up as an option because it's the only way to warn without counting the answer wrong.  Plus I don't know, maybe you like those mascot things.

The other option uses the "almost right" feedback interface, like simplification reminders do.  Which means it only warns you after counting the problem wrong.