Implement `#[key = ...]`

This PR implements the ability to use a limited set of literals for keys when serializing and deserializing structs. The motivation is written in https://github.com/serde-rs/serde/issues/2695.

Closes https://github.com/serde-rs/serde/issues/2695

Limitations

We purposefully error for string keys since this is already supported using rename.
Floats are not supported (Rust doesn't support them for match statements and in practice, this is not done).
Negative keys are not supported for the time being.
Keys can clash if the user wants to. (Footgun?)

serde-rs / serde

Implement `#[key = ...]` #2698

Limitations