uses two branches of oxigraph crates

[jonassmedegaard]

This project uses both stable crate oxigraph v0.3 and alpha versions of underlying crates oxrdf etc.

I think it is better to move to the newer oxigraph also for the main crate, to avoid duplicate code (and to ease packaging for Debian, which provides only the newer oxigraph v0.4 by now).

Here is my attempt at modernizing, in case that is helpful - it compiles, but I am uncertain if the result is functionally correct.

[labra]

Done at PR #86