Reimplement IriHandler to use StringHasher

The handler contained the same information three times:
* Concatenating `scheme` + `':'` + `hierp` + `'?'` + `query` + `'#'` + `fragment` is the same as `iri`
* `json` was a single element array with the `iri`.

None of these fields was indexable!

Simple solution: Reuse the string logic.