Add c14n for node and document

[saks]

There are not too many choices when it comes to "Canonical XML" implementation for rust. This PR is a compilation of ideas, approaches and test files from the following projects:

• https://gitlab.com/reinis-mazeiks/xml_c14n
• https://github.com/sparklemotion/nokogiri

Work items

• add canonicalize method for Node and Document
• add Node::ancestors method (I personally find it very useful when using "nokogiri", plus it help in tests a lot). Although I don't feel too strongly about, can remove if it doesn't fit.
• add Node::at_xpath method. Yes, it is similar to findnodes (and can ultimately be implemented using Context), but with design from "nokogiri", I find it a lot more flexible when navigating a tree. Same as for the previous item, can update PR to remove it if necessary.

Any feedback is welcome! I don't feel particularly attached to any approach here. Please let me know how to make this PR better.

[dginev]

@saks interesting, there is a very high-level question I have first.

Why do you think this wrapper crate is the right place to introduce this method?
rust-libxml mostly tries to follow the libxml2 coverage, and probably should not grow much further, as a general-purpose XML utility library.

As you mention xml_c14n exists as a separate Rust crate. Maybe your work could also be packaged as a separate crate?