Rename fields at runtime

[pronebird]

I'd love to see configurable serializers and deserializers that aren't baked into the struct.

I have a few structs that are shared between Rust crates, which typically prefer snake_case and interop with other languages that prefer camelCase.

Languages like Swift use separate encoders that can be configured to modify the key coding strategy without modifying the struct or duplicating it. I think it would be great to have something similar in serde.