This would make more sense in user code:
```rs
Yaml::load_from_str("foo"); // Explicit that we're parsing YAML
load_from_str("foo"); // Too implicit, too generic, may be from another
lib
```
Plus, this mirrors `MarkedYaml`'s behavior.
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.
* Make `YamlLoader` generic on the type of the `Node`. This is required
because deeper node need to have annotations too.
* Add a `LoadableYamlNode` trait, required for YAML node types to be
loaded by `YamlLoader`. It contains methods required by `YamlLoader`
during loading.
* Implement `LoadableYamlNode` for `Yaml`.
* Take `load_from_str` out of `YamlLoader` for parsing non-annotated
nodes. This avoids every user to specify the generics in
`YamlLoader::<Yaml>::load_from_str`.
