Add TLS support (#81)

When `-- features tls` is specified for tarpc, RPC communication can
also occur over a `TlsStream<TcpStream>` instead of a `TcpStream`.

* The functional tests have been refactored to use a common set of
functions for constructing the client and server structs so that all
the tests are shared across non-tls and tls test runs.

* Update pre-push to test TLS

* The `cfg_attr` logic caused many false warnings from clippy, so for now the crate docs for TLS are not tested.

hooks/pre-push

@@ -70,10 +70,12 @@ run_cargo() {
                     rustup run $2 cargo $1 &>/dev/null
                 else
                     rustup run nightly cargo $1 --features unstable &>/dev/null
+                    rustup run nightly cargo $1 --features unstable,tls &>/dev/null
                 fi
 	else
 		printf "${PREFIX} $VERB... "
 		cargo $1 &>/dev/null
+		cargo $1 --features tls &>/dev/null
 	fi
 	if [ "$?" != "0" ]; then
 		printf "${FAILURE}\n"