codec: move into tokio-util (#1675)

Related to #1318, Tokio APIs that are "less stable" are moved into a new
`tokio-util` crate. This crate will mirror `tokio` and provide
additional APIs that may require a greater rate of breaking changes.

As examples require `tokio-util`, they are moved into a separate
crate (`examples`). This has the added advantage of being able to avoid
example only dependencies in the `tokio` crate.

tokio-util/src/lib.rs

@@ -0,0 +1,17 @@
+#![doc(html_root_url = "https://docs.rs/tokio-util/0.2.0-alpha.6")]
+#![warn(
+    missing_debug_implementations,
+    missing_docs,
+    rust_2018_idioms,
+    unreachable_pub
+)]
+#![deny(intra_doc_link_resolution_failure)]
+#![doc(test(
+    no_crate_inject,
+    attr(deny(warnings, rust_2018_idioms), allow(dead_code, unused_variables))
+))]
+
+//! Utilities for working with Tokio.
+
+pub mod codec;
+pub mod udp;