Move enum fields into structs

[adamchalmers]

This is purely a refactor, I don't expect any behavioural changes.

Purpose of the refactor is: Bon (our builder API macro crate) doesn't work on enums. But some enums have struct-like named fields, making them very close to a struct and therefore having all the same problems, e.g. when you add a field, it'll cause compile errors in the engine, forcing people to handle those new fields.

Solution is to move those enum fields into a new struct, which CAN have the Builder derive on it.

This is going to make the new Face API easier to integrate by causing fewer compile errors. Moving these fields into named structs will be a breaking change, but it's a very easy breaking change, and will reduce future changes.

Also incidentally bump rustls for a security patch.

[codecov]

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[iterion]

iiuc this is a breaking change for the Rust types but not for the API itself, and the api.json suggests the same