Intern strings for Pid components

[andrewjstone]

We must be able to serialize and deserialize interned strings to allow distribution. This may result in the need to implement Serializeand Deserialize manually for Pid if the interner doesn't implement those traits. We will just send the strings themselves over the wire, but preferably packed into a single allocated Vec to minimize size and allocations. This may be reason enough to implement Serialize and Deserialize manually rather than using the implementations of a given interner. If we wanted to get even more fancy, we could pack an entire Envelope into a single Vec as well.