Implement Encode using ProtoEncode.

First step towards hiding ProtoEncode as an implementation detail and
only exposing Encode externally.

src/proto/base_codec.rs

@@ -162,6 +162,8 @@ where
 /// This trait is implemented by types that can be encoded into messages with
 /// a `ProtoEncoder`.
 /// Only here to enable `ProtoEncoder::encode_vec`.
+/// Attempting to implement Encode for Vec<T> otherwise fails because none of
+/// the types involved are defined in this crate.
 pub trait ProtoEncode {
     /// Attempts to encode `self` with the given encoder.
     fn encode(&self, encoder: &mut ProtoEncoder) -> io::Result<()>;
@@ -308,6 +310,12 @@ impl<T: ProtoEncode> ProtoEncode for Vec<T> {
     }
 }
 
+impl<'a, T: ProtoEncode> Encode<&'a T> for BytesMut {
+    fn encode(&mut self, value: &'a T) -> io::Result<()> {
+        value.encode(&mut ProtoEncoder::new(self))
+    }
+}
+
 /*=======*
  * TESTS *
  *=======*/