Derive Macro BitsCast 

#[derive(BitsCast)]
{
    // Attributes available to this derive:
    #[bitfld]
}

Derive BitsCast (for enum only right now)

Inside usage but not recommanded:

#[derive(BitsCast, Clone, Copy)] // must derive `Clone` and `Copy` too
#[bitfld(u8)] // must has helper attribute
enum {...}

Recommand usage:

#[bitfld(u8)] // attribute macro `bitfld` will auto expand to correct usage
enum {...}