Reduce number of packages

[Gozala]

I think we could trim number of packages we have here:

1. Principal is pretty standalone and I think I'd like to keep it that way.
2. core and validator could probably merge I don think there's point to have them separate
   • initially client did not use validator which is why it was separate, but it's no longer the case because we use capabilities to create invocations / delegations these days
3. Transport is not that obvious. Perhaps some parts could merge into server and other into client. It was split out because idea is that you should be able to bring your own transport, so maybe it's better to leave it as is for now

Alternatively we may consider an umbrella package(s) that just re-exports all the relevant bits from one so you don't have to pull bunch of them yourself