Serde Versions Derive

serde_versions_derive exports an attribute macro that adds versioning support for structs.

When serializing a named field struct it will automatically add a new field containing the version. It also allows deserializing the versioned type directly back to the unversioned one.

Under the hood this works by creating a new struct that wraps the original struct plus adds a version byte field.

Internally this new struct uses #[serde(flatten)] to serialize as expected. The original struct uses #[serde(to, from)] to add the version field when serializing and remove it when deserializing.

usage:

```rust

[version(3)]

[derive(Clone, Serialize, Deserialize)]

struct S { i: i32, } ```

This produces the following ```rust

[derive(Clone, Serialize, Deserialize)]

[serde(into = "Sv3", from = "Sv3")]

struct S { i: i32, }

[derive(Clone, Serialize, Deserialize)]

struct _Sv3 { version: u8, #[serde(flatten)] inner: S }

// plus implementations of To, From and to_versioned() for S ```

and will Serialize to:

json { "version": 3, "i": 0 }

Note due to limitations of #[serde(to, from)] this does not support structs with type parameters.