Review whether polymorphic relation interface is required

[lindyhopchris]

At the moment we have a Contracts\Schema\PolymorphicRelation interface, with a inverseTypes() method on it.

This is actually redundant as we have added the allInverse() method to the Relation contract. We can probably just remove the polymorphic relation interface and add isPolymorph() and isNotPolymorph() helper methods to the relation interface.

So creating this issue as a reminder to review whether we do actually need to the seperate interface as it would be good to remove it during the beta series if it is no longer required.

[lindyhopchris]

See #59 as these two issues should be dealt with together.

[lindyhopchris]

Removed from milestone as deferring this to a subsequent beta release.