XML & YAML support

[01mf02]

This is work in progress.

TODO:

• [ ] Document --from and --to in help.txt
• [ ] Document YAML behaviour (supported tags !!int etc., keys must be strings limitation)
• [x] Use new saphyr-parser version once https://github.com/saphyr-rs/saphyr/issues/50 is closed
• [ ] Implement fromxml, toxml, fromyaml, toyaml
• [ ] Encapsulate lexer errors in newtype to ease updates
• [ ] Recover shared YAML values
• [ ] Write parsing tests