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/encoder.rs

@@ -0,0 +1,22 @@
+use bytes::BytesMut;
+use std::io;
+
+/// Trait of helper objects to write out messages as bytes, for use with
+/// `FramedWrite`.
+pub trait Encoder {
+    /// The type of items consumed by the `Encoder`
+    type Item;
+
+    /// The type of encoding errors.
+    ///
+    /// `FramedWrite` requires `Encoder`s errors to implement `From<io::Error>`
+    /// in the interest letting it return `Error`s directly.
+    type Error: From<io::Error>;
+
+    /// Encodes a frame into the buffer provided.
+    ///
+    /// This method will encode `item` into the byte buffer provided by `dst`.
+    /// The `dst` provided is an internal buffer of the `Framed` instance and
+    /// will be written out when possible.
+    fn encode(&mut self, item: Self::Item, dst: &mut BytesMut) -> Result<(), Self::Error>;
+}