codec: Add Encodable, Decodable interfaces

codec/decode.go

@@ -2,6 +2,14 @@ package codec
 
 import "io"
 
+// Decodable is any type that can decode itself from a decoder.
+type Decodable interface {
+	// Decode reads data from decoder, replacing the data of the object. It
+	// returns the amount of bytes written, and an error if the write
+	// stopped early.
+	Decode(decoder *Decoder) (n int, err error)
+}
+
 // Decoder wraps an [io.Reader] and decodes data from it.
 type Decoder struct {
 	io.Reader

codec/encode.go

@@ -2,6 +2,13 @@ package codec
 
 import "io"
 
+// Encodable is any type that can write itself to an encoder.
+type Encodable interface {
+	// Encode sends data to encoder. It returns the amount of bytes written,
+	// and an error if the write stopped early.
+	Encode(encoder *Encoder) (n int, err error)
+}
+	
 // Encoder wraps an [io.Writer] and encodes data to it.
 type Encoder struct {
 	io.Writer