ttrpc: add required codec.go

Signed-off-by: Stephen J Day <stephen.day@docker.com>

codec.go

@@ -0,0 +1,26 @@
+package ttrpc
+
+import (
+	"github.com/gogo/protobuf/proto"
+	"github.com/pkg/errors"
+)
+
+type codec struct{}
+
+func (c codec) Marshal(msg interface{}) ([]byte, error) {
+	switch v := msg.(type) {
+	case proto.Message:
+		return proto.Marshal(v)
+	default:
+		return nil, errors.Errorf("ttrpc: cannot marshal unknown type: %T", msg)
+	}
+}
+
+func (c codec) Unmarshal(p []byte, msg interface{}) error {
+	switch v := msg.(type) {
+	case proto.Message:
+		return proto.Unmarshal(p, v)
+	default:
+		return errors.Errorf("ttrpc: cannot unmarshal into unknown type: %T", msg)
+	}
+}