Start work on an IEqualityComparer<> implementation... but gave up.

Unfortunately, this simply is not possible using expression trees, so I'd need to completely reimplement what I have
with manual IL generation (urgh) and all its warts, specifically surrounding dealing with both value and reference types.

Alternatively, I just just wrap the generated Func<>'s in an object that just defers to them, but of course that's considerably slower.