Switch from sax-rs to rust-xml

Remove `sax-rs` in favor of a pure Rust implementation named `rust-xml`.

**Do not merge for the moment** as this issue needs to be solved before it compiles: https://github.com/netvl/rust-xml/issues/3

The khronos XML file has an attribute that contains a `;` and that makes `rust-xml` trigger an error (see issue above). If I manually remove this `;` the library compiles and works perfectly.

Closes #111
This PR should not conflict with #119