Canonical metadata

[Zheoni]

This PR adds support for all the canonical metadata keys. I added a bunch of methods that will help the conversions from a YAML value to a known data structure/type.

The basic usage is something like

use cooklang::metadata::CooklangValueExt; // trait that adds more conversions to a YAML value

let metadata = &recipe.metadata;
let course = metadata.get(StdKey::Course)?.as_str()?;
let tags = metadata.get(StdKey::Tags)?.as_tags()?;

When the expected type is clear I added a more direct method. For example, for tags you can also do:

let tags = metadata.tags()?;

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Time can now be always parsed from HhMm like the spec says. No spaces.

Also, when time is used with cook time and prep time, time always takes precedence. I know I asked in the discord but the serde_yaml::Mapping API made it difficult to implement last one gets preference, and this is much simpler. The warning is emitted telling the user prep time/cook time has been overriden so I think it's no big problem.

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I dind't merge it myself just to confirm the API is ok and that I didn't missinterpret the spec.