Derivation of Serialize and Deserialize that replaces struct keys with numerical indices.

Primary use case is to handle CTAP CBOR messages, in particular support for:

• skip_serializing_if for optional keys
• configurable index offset

#[derive(Clone, Debug, PartialEq, SerializeIndexed, DeserializeIndexed)]
#[serde_indexed(offset = 1)]
pub struct SomeKeys {
    pub number: i32,
    #[serde(skip_serializing_if = "Option::is_none")]
    pub option: Option<u8>,
    pub bytes: [u8; 7],
}

This was a nice opportunity to learn proc-macros, I roughly followed serde-repr.

To see some generated code, run cargo expand --test basics.

^{serde-indexed is licensed under either of Apache License, Version 2.0 or MIT License at your option.}
_{Unless you explicitly state otherwise, any contribution intentionally submitted for inclusion in the work by you, as defined in the Apache-2.0 license, shall be dual licensed as above, without any additional terms or conditions.}