Allow (un)packing structs containing Vec<T> with #[derive(MsgPacker)]

[aurelj]

Currently the following code doesn't work:

#[derive(MsgPacker)]
pub struct A {
    b: Vec<u32>,
}

because Vec<T> doesn't implement Packable.

This PR changes the binary (un)packer to apply to a new MsgPackerBin wrapper type used to mark Vec<u8> that should be encoded as msgpack binary format.

This free the way to implement (Un)Packable for Vec<T> and thus allows Vec<T> to be packed as msgpack array format, fixing the above mentioned code.