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 you can't warn the other way (via the "almost right" feedback interface a la simplification reminders) without counting the answer wrong.

By the way, testing this functionality isn't straightforward because the exercises this is useful for aren't in the repo. I tested by setting options.simplify = false; right after the options declaration for expression answers, around line 1825 in answer-types.js. This lets you pull up the exercise adding_and_subtracting_complex_numbers and you test by typing in answers unsimplified that make use of x for *. e.g. 1+ix1, for 1+i