Provide access to the prefix/namespace map

[phillord]

Currently, the RDF parser gives no access to namespaces or prefixes, which makes it hard to roundtrip RDF from file to file!

[pchampin]

Renaming this issue, as it is not specific to XML (the same is true of Turtle or TriG.

[pchampin]

I see that https://github.com/oxigraph/rio/releases/tag/v0.7.1 addresses it for the Turtle-family of parsers. Nice! ❤️