Add ability to serialize replies and events

[notgull]

I was writing a test harness for breadx, where I serialize the requests it's supposed to send into a byte stream and then compares it to make sure it's outputting the right stuff. I'd also like to make it so I could feed the test harness replies, events and errors. However, to do this, I would need to be able to serialize these items. At the moment, replies and events do not implement Serialize.

Is there any particular reason why this is? If not, I'll go ahead and write a PR that adds these impls to these objects.

[psychon]

Is there any particular reason why this is?

I cannot think of any. I am sure that somewhere there was already some talk about it, but I do not know any details. Perhaps there was something about the special case of "most" events being 32 bytes? Dunno...

So... feel free & remember that FDs make everything complicated. :-)