Bump serde_yaml to 0.9

[fennewald]

• Bumps serde_yaml to 0.9, the most recent release
• Bumps format_serde_error to 0.3.1 from 0.3.0
• Updated a few tests with the new expected output

[fennewald]

Just noticed PR 22, in it's current form this PR ignores 0.8 compatibility. Given the slightly different outputs, the only solution I can imagine would involve gating serde_yaml versions behind a feature flag (e.g. serde_yaml_0.8, and serde_yaml_0.9 feature flags. This feels verbose.

[dsully]

I believe I'm encountering this error due to the older version of serde_yaml as well:

Also with the recent serde_yaml deprecation, there are a few replacements that should be allowed IMO.

Thanks!