Complain when adding conflicting identified resources

[Julian]

Either by default or when using a stricter wrapper object, we should have a way to complain if a resource is being added in a conflicting way -- i.e. if two different resources have the same URI.

So:

registry.with_resource("http://example.com", Resource.opaque(12))
registry.with_resource("http://example.com", Resource.opaque("foo"))

should raise some conflicting resource error.

The same should happen when calling registry.combine on multiple registries -- though here we should be careful because downstream (e.g. in jsonschema) a use of combining registries is to add "known" schemas to some additional ones, and sometimes the combining is to allow users to override schemas for whatever purpose. Perhaps we could explicitly force such registries to first drop the known resources before combining.

[Julian]

I've gone back and forth on whether to actually implement this or not (and allow this). I think by now it hasn't overwhelmingly convinced me that this is necessary, so let's just close. Someone who wants to guard against this of course can explicitly check before appending a new resource.