c14n and more bindgen headers

[0.3.6] (2025-13-07)

Added

• c14n, thanks to @saks and @tstenner
  • add canonicalize method for Node and Document
  • see tests/c14n.rs for examples
• Utility node methods, thanks to @saks (nokogiri inspired)
  • add Node::ancestors method
  • add Node::at_xpath method. Similar to findnodes but nokogiri inspired.

This work still hasn't been extended to RoNode, contributions welcome.

Changes

• Added more low-level headers from bindgen for wrapper uses interested in unsafe low-level libxml2 calls.
  The bindgen coverage has been in flux since the v0.3.4 release, as we stabilize the new build approach.