put original id stx in log message rather than datum

[jbclements]

This change puts the original syntax object into the log message rather than the corresponding datum, which allows the receiver of the log message to see what source location was mutated.

It's possible that if this change makes sense, there are a bunch of other changes that would follow, for things other than ids; let me know if that's the case, I'd be happy to broaden the net a bit. Also, maybe I should include tests?

Basically, let me know if you think this is a reasonable direction to go in.

[LLazarek]

Seems like a good direction to me! It probably makes the most sense to change both the "new" and "original" parts of the message to syntax objects, rather than just the "original". If only for consistency's sake, since that's what the rest of the mutators do and what the docs claim (oops).

The only additional place I can think of to do this as well would be for constant mutators, which have pretty much the same pattern. And a few tests would be great -- perhaps those could use a small extension of test-logged-mutation from testing-util.rkt.