Support Selector prefix for enum

[crab2313]

I have a use case to parse a small packet to enum. The selector is the first byte. From the documentation I must implement something like:

#[derive(Nom)]
struct PacketParser {
    code: u8,
    #[nom(Parse = "{ |i| PacketType::parse(i, code) }")]
    packet: PacketType,
}

#[derive(Nom)]
enum PacketType {
    ....
}

It's very hard to handle the enum nested in enum case due to the selector introduced in the parse function prototype. I need a way to parse PacketType and specifying the Selector is the first byte. And PacketType::parse don't need the second parameter as the selector since the information is encoded in the first byte of packet.