Change #== dependency from #coerce to #equalizer_coerce

[backus]

I have an object which defines a #coerce instance method as part of its public API. I didn't realize until recently that this breaks #== for this object and all of the objects that own it until today.

I understand after talking to @dkubb that this is a weak convention in ruby, but it might make sense to deprecate the use of #coerce and instead depend on a method that is less likely to naturally collide with unrelated method names that are not meant for equalizer