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.

src/stream_type.rs

@@ -0,0 +1,55 @@
+use std::io;
+use tokio_core::io::Io;
+use tokio_core::net::TcpStream;
+#[cfg(feature = "tls")]
+use tokio_tls::TlsStream;
+
+#[derive(Debug)]
+pub enum StreamType {
+    Tcp(TcpStream),
+    #[cfg(feature = "tls")]
+    Tls(TlsStream<TcpStream>),
+}
+
+impl From<TcpStream> for StreamType {
+    fn from(stream: TcpStream) -> Self {
+        StreamType::Tcp(stream)
+    }
+}
+
+#[cfg(feature = "tls")]
+impl From<TlsStream<TcpStream>> for StreamType {
+    fn from(stream: TlsStream<TcpStream>) -> Self {
+        StreamType::Tls(stream)
+    }
+}
+
+impl io::Read for StreamType {
+    fn read(&mut self, buf: &mut [u8]) -> io::Result<usize> {
+        match *self {
+            StreamType::Tcp(ref mut stream) => stream.read(buf),
+            #[cfg(feature = "tls")]
+            StreamType::Tls(ref mut stream) => stream.read(buf),
+        }
+    }
+}
+
+impl io::Write for StreamType {
+    fn write(&mut self, buf: &[u8]) -> io::Result<usize> {
+        match *self {
+            StreamType::Tcp(ref mut stream) => stream.write(buf),
+            #[cfg(feature = "tls")]
+            StreamType::Tls(ref mut stream) => stream.write(buf),
+        }
+    }
+
+    fn flush(&mut self) -> io::Result<()> {
+        match *self {
+            StreamType::Tcp(ref mut stream) => stream.flush(),
+            #[cfg(feature = "tls")]
+            StreamType::Tls(ref mut stream) => stream.flush(),
+        }
+    }
+}
+
+impl Io for StreamType {}