Add support for yaml parsing/rendering.

[toots]

This PR adds support for YAML parsing. It uses the the yaml module, which has a great JSON<->Yaml equivalence.

Therefore, YAML parsing uses the same infrastructure as JSON parsing, i.e.:

  let yaml.parse (x : {
    bla: string,
    baz: float,
    key_with_methods: string,
    record: { a: float, b: string},
  }) = yaml

It would be nice to include this by default but, ala, the yaml module is using a libffi binding over the libyaml library and this would be adding a hard dependency on an external library..

TODO:

• Doc!
• Window support in the CI The yaml module needs some work to be able to cross-compile. See: Embeds a modified copy of libyaml avsm/ocaml-yaml#51

Fixes: #2853