implementation of URDNA2015 in Rust

Hi all,

I have just implemented the c14n algorithm in Rust [1]. It went quite 
smoothly, kudos to the editors for a very clear spec :) I have a few 
comments that I'll write down later, but overall, I did not run into any 
major problem.

This is a quick and dirty implementation, but it gives the correct 
results for the first two examples of the spec. I still need to 
implement some code to run the test suite.

   best

[1] 
https://github.com/pchampin/sophia_rs/commit/15d8183ce00f0a23d8174d9dc9ad0b06f88fc3cc