Iri<T> and IriRef<T> should implement Debug regardless of T

[pchampin]

Currently Iri<T> and IriRef<T> derive Debug, meaning that they only implement it if T implements it.

If would make more sense for Iri and IriRef to provide their own implementation of Debug, regarless of T (after all, T implements Borrow<str> so it is trivially "displayable").

This would also avoid the frustration of Iri<T> or IriRef<T> not implementing Term when T is too generic (see #169).