Disallow binding multiple signals to the same property

This is a breaking change, but binding two signals to the same property was never a good idea, since it's undefined which signal would affect the property when (like if they both sent values in the same run loop iteration).

I've added checking for Debug builds which should help catch (some) programmer errors, but it's not comprehensive. The semantics haven't changed for Release at all.

CC @joshvera @andymatuschak