feat: reducing allocations in the 9p protocol layer

[sminez]

Making use of a shared mutable buffer between 9p parsing Coros and the logic running them in the sans-io layer so that we only need to allocate and re-use a single buffer for parsing messages rather than allocating a new Vec for each read.

The resulting public API is a little odd as it means that consumers of the crate will not be able to implement their own IO wrappers unless the methods on SharedBuf are made public. Until there is a use case for that however, I am planning on leaving things this way so that consumers of ninep aren't able to footgun with the unsafe API that SharedBuf exposes.