Introduce #[derive(CgpData)] and #[derive(CgpRecord)] Macros

[soareschen]

Summary

This PR introduces the macros #[derive(CgpData)] and #[derive(CgpRecord)] to simplify the deriving of extensible data traits. The macros are shortcuts for the following:

• #[derive(CgpRecord)] is the same as #[derive(HasField, HasFields, BuildField)]
• #[derive(CgpVariant)] is the same as #[derive(FromVariant, HasFields, ExtractField)]
• #[derive(CgpData)] is the same as deriving CgpRecord or CgpVariant, depending on whether the body is a struct or enum.

Example

Given the following:

#[derive(CgpData)]
pub struct Person {
    pub name: String,
    pub age: u8,
}

#[derive(CgpData)]
pub enum User {
    Known(Person),
    Anonymous(u64),
}

is the same as:

#[derive(HasField, HasFields, BuildField)]
pub struct Person {
    pub name: String,
    pub age: u8,
}

#[derive(FromVariant, HasFields, ExtractField)]
pub enum User {
    Known(Person),
    Anonymous(u64),
}