codec: update to use std-future (#1214)

Strategy was to

- copy the old codec code that was temporarily being stashed in `tokio-io`
- modify all the type signatures to use Pin, as literal a translation as possible
- fix up the tests likewise

This is intended just to get things compiling and passing tests. Beyond that there is surely
lots of refactoring that can be done to make things more idiomatic. The docs are unchanged.

Closes #1189

tokio-codec/src/encoder.rs

@@ -0,0 +1,25 @@
+use bytes::BytesMut;
+use std::io;
+
+/// Trait of helper objects to write out messages as bytes, for use with
+/// `FramedWrite`.
+
+// Note: We can't deprecate this trait, because the deprecation carries through to tokio-codec, and
+// there doesn't seem to be a way to un-deprecate the re-export.
+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>;
+}