Change module structure. (#122)

* Change `client::{future, sync}, server::{future, sync}` to `future::{client, server}, sync::{client, server}`

This reflects the most common usage pattern and allows for some types to not need to be fully qualified when used together (e.g. the previously-named `client::future::Options` and `server::future::Options` can now be `client::Options` and `server::Options`).

* sync::client: create a RequestHandler struct to encapsulate logic of processing client requests.

The largest benefit is that unit testing becomes easier, e.g. testing that the request processing stops when all request senders are dropped.

* Rename Serialize error variants to make sense.

* Rename tarpc_service_ConnectFuture__ => Connect (because it's public)

* Use tokio proto's ClientProxy.

Rather than reimplement the same logic. Curiously, this commit
isn't a net loss in LOC. Oh well.

* Factor out os-specific functionality in listener() into their own fns

* Remove service-fn dep

src/sync/mod.rs

@@ -0,0 +1,4 @@
+/// Provides the base client stubs used by the service macro.
+pub mod client;
+/// Provides the base server boilerplate used by service implementations.
+pub mod server;