Stable way to decl enums tag

[Frago9876543210]

stable rust:

#[derive(Io)]
#[endian(default_endian)] //only for Prim
#[repr(Prim)] //u32, i32, ...
enum Packet {
    #[tag(0xdeadbeef)]
    Foo(type: Io),
}

nightly rust: (WIP: enums branch)

#[derive(Io)]
#[endian(default_endian)] //only for Prim
#[repr(Prim)] //u32, i32, ...
enum Packet {
    Foo(type: Io, type1: Prim, ...) = 0xdeadbeef,
}

[Frago9876543210]

TODO:

• Stable macro for struct-like & tuple enums
• Custom repr types (#[wrapper(type)])
• Zero cost write for unit enums? (enum as <repr_ty>)
• More tests