sync: move into tokio crate (#1705)

A step towards collapsing Tokio sub crates into a single `tokio`
crate (#1318).

The sync implementation is now provided by the main `tokio` crate.
Functionality can be opted out of by using the various net related
feature flags.

tokio/tests/sync_errors.rs

@@ -0,0 +1,29 @@
+#![warn(rust_2018_idioms)]
+
+fn is_error<T: ::std::error::Error + Send + Sync>() {}
+
+#[test]
+fn mpsc_error_bound() {
+    use tokio::sync::mpsc::error;
+
+    is_error::<error::SendError>();
+    is_error::<error::TrySendError<()>>();
+    is_error::<error::UnboundedRecvError>();
+    is_error::<error::UnboundedSendError>();
+    is_error::<error::UnboundedTrySendError<()>>();
+}
+
+#[test]
+fn oneshot_error_bound() {
+    use tokio::sync::oneshot::error;
+
+    is_error::<error::RecvError>();
+    is_error::<error::TryRecvError>();
+}
+
+#[test]
+fn watch_error_bound() {
+    use tokio::sync::watch::error;
+
+    is_error::<error::SendError<()>>();
+}