`Packets` trait

[BenLeadbetter]

Is would be good to implement a Packets trait for midi2 message wrappers.

Something like the following:

pub struct PacketsIterator<'a>;

impl<'a> core::iter::Iterator for PacketsIterator<'a> {
    type Item = &'a [u32];
    ...
}

pub trait Packets<B: Ump>: Data<B> {
    fn packets(&self) -> PacketsIterator;
}

The trait could potentially even be implemented via a blanket impl for all messages which also implement Data.

impl<B: Ump, M: Data<B>> Packets<B> for M {
    fn packets(&self) -> PacketsIterator { ... }
}