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.
* 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`.
Change scanner's complex test:
```diff
- *coffee:
+ *coffee :
amount: 4
- *cookies:
+ *cookies :
amount: 4
```
According to https://play.yaml.io/main/parser, this example was invalid
in the first place. Adding a space makes it so that the colon is not
part of the alias name.
Also fix colons not being able to be part of anchors.
