Fail to compile if classes have filters that cannot be diffed

[wolfsage]

If a class specifies a filter that doesn't:

• match an ix property on the class
• provide its own differ subroutine
• refer to a joined relationship

then fail to compile mentioning why.

This saves consumers trouble down the road if they implement a filter that doesn't match up to an Ix property but don't provide a custom differ. When this would happen, Ix would crash during ix_get_list_updates() when trying to compare rows against filters.

[rjbs]

Cool, works for me. I briefly thought, "this will slow down startup," but we're using Moose. ;)