Several improvements around Bytesable and Message.

[frankmcsherry]

These commits do some likely uncontroversial tidying, and then the somewhat controversial removal of the Bundle type which communicates a preference for using bincode on Message. Although we started to generalize the allowed serialization strategies, Bundle has been in the public interface for ParallelizationContract, and generally there needs to be some structure to allow the various helpers to pull out timestamps, etc. This shift does not remove flexibility that actually existed, as far as I can tell.

The intended direction is to replace the Bytesable implementation for Message, which in this PR looks like so

impl<T, C> crate::communication::Bytesable for Message<T, C>
where
    T: Serialize + for<'a> Deserialize<'a>,
    C: Serialize + for<'a> Deserialize<'a>,
{
...

with one for which we have only C: Bytesable. This allows the container to specify its strategy for serialization. There are a few ways to do this, and I haven't figured out which one is best or worst or .. whatever. That part being tbd, I thought I'd put this up for review in the meantime.