merino/relevancy/suggest: Refactorings for uniffi_parse_rs

[bendk]

I'm working on a new UniFFI parser
(mozilla/uniffi-rs#2841) and it currently has a couple extra requirements for types used in exported functions:

• The types are publicly available from other crates.
• The name matches the one used in the custom_type! macro.
• Macro expansion is not supported, the types need to be written out by hand.

I could maybe rework the parser to avoid these requirements, but the first two the changes feel like improvements to me. Not being able to use macros for UniFFIed types is worse but I don't think it will affect us all that much in practice. Please push back if you don't think so.

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[bendk]

I'm going to be opening several of these issues. I feel like this one is the most controversial since the "no macros" rule is a real limitation. Please tell me if you think that's going to negatively affect you too badly.