update serde-yaml to 0.9

[arcnmx]

The update breaks configurations because of dtolnay/serde-yaml#342 - the suggestion may need to be used with a special with wrapper to work with Vec<Enum> and other container types 😞
(the better approach may be to bake it into the Deserialize impl somehow?)

The issue here is that with the update the config is no longer compatible with json parsed as yaml, as it will then rely on the yaml-exclusive !tag feature. Alternatively, just switch to serde-json instead and give up on user-readable configs? Or support both via file extension and/or other option?

EDIT: singleton_map_recursive is what you actually want, to work around Vec issues