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/codec/macros.rs

@@ -0,0 +1,7 @@
+/// A macro to reduce some of the boilerplate for projecting from
+/// `Pin<&mut T>` to `Pin<&mut T.field>`
+macro_rules! pin {
+    ($e:expr) => {
+        std::pin::Pin::new(&mut $e)
+    };
+}