Implement #[derive(Packable)] #11362

For a lot of structs we need an implementation of both the Serialize, Deserialize and Packable traits but the implementation of Packable trait is equivalent to Serialize and Deserialize as those are already optimal serialization-size wise.

In those situation having an explicit implementation of Packable is a lot of boilerplate and hence it would be desirable to reduce that by having a #[derive(Packable)] macro that would generate:

impl<let MY_STRUCT_SERIALIZED_LENGTH: u32> Packable<MY_STRUCT_SERIALIZED_LENGTH> for MyStruct
{
    fn pack(self) -> [Field; MY_STRUCT_SERIALIZED_LENGTH] {
        self.serialize()
    }

    fn unpack(packed: [Field; MY_STRUCT_SERIALIZED_LENGTH]) -> Self {
        Self::deserialize(packed)
    }
}