A few changes have had to be made to `LoadableYamlNode`:
* The `From<Yaml>` requirement has been removed as it can be
error-prone. It was not a direct conversion as it is unable to
handle `Yaml::Hash` or `Yaml::Array` with a non-empty array/map.
* Instead, `from_bare_yaml` was added, which does essentially the same
as `From` but does not leak for users of the library.
* `with_marker` has been added to populate the marker for the `Node`.
The function is empty for `Yaml`.
`load_from_*` methods have been added to `MarkedYaml` for convenience.
They load YAML using the markers.
The markers returned from `saphyr-parser` are not all correct, meaning
that tests are kind of useless for now as they will fail due to bugs
outside of the scope of this library.
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