Port to libchan, pt. 2

- Core messaging types/functions (Message, Sender, Receiver, Object, Server, Handler, Pipe, Copy) moved to **libswarm** top-level package.
- Helper types/functions (NopSender/Receiver, Encode/DecodeStream, Task, Tree) moved to **utils** package.
- Object renamed to Client
- `Obj(sender)` renamed to `AsClient(sender)`. Still not convinced about the name. Suggestions invited - it needs to communicate that you're wrapping a Sender so that you can use the higher-level Client API on it.