[PERFORMANCE] non-GPL format checker for `iri` and `iri-reference` utterly slow

[jkowalleck]

the non-GPL format checker for iri and iri-reference is utterly slow.

the respective format checker library rfc3987-syntax was introduced via #1388

It takes about one minute on my machine to validate a JSON with ~1800 iri-references.

[jkowalleck]

I am working on this.
possible solutions:

• find a non-GPL replacement lib
• improve the respective lib