Refs equality: remove IvoryEq instance.

  * We shouldn't be comparing references for equality.  Makes accidently comparing references rather than values a compile-time bug.

  * Must have pointers be an instance of IvoryEq, since we have to compare to null.

  * Add a runtime check for the use of refcpy to check for equality before copying.  If they are equal, we do nothing.
    * This means that a potentially unintended copy fails silently.