fix: more generic BlockCodec, remove encoder & decoder props

This is what we probably should have done to solve the conflict in cea9063
because we ended up with a `BlockCodec` that has too many properties and is
therefore not very useful.

This makes `BlockCodec` just an implementer of both `BlockEncoder` and
`BlockDecoder` but `codec()` returns you that, plus `encoder` and `decoder`
properties so you have all the things and can consume them however you like.

Ref: ipld/js-dag-cbor#18 (comment)

src/codecs/codec.js

@@ -1,16 +1,31 @@
 // @ts-check
 
+/**
+ * @template {number} Code
+ * @template T
+ * @typedef {import('./interface').BlockCodec<Code, T>} BlockCodec
+ */
+/**
+ * @template {number} Code
+ * @template T
+ * @typedef {import('./interface').BlockEncoder<Code, T>} BlockEncoder
+ */
+/**
+ * @template {number} Code
+ * @template T
+ * @typedef {import('./interface').BlockDecoder<Code, T>} BlockDecoder
+ */
+
 /**
  * @template {string} Name
  * @template {number} Code
  * @template T
- *
  * @param {Object} options
  * @param {Name} options.name
  * @param {Code} options.code
  * @param {(data:T) => Uint8Array} options.encode
  * @param {(bytes:Uint8Array) => T} options.decode
- * @returns {import('./interface').BlockCodec<Code, T>}
+ * @returns {BlockCodec<Code, T> & { encoder: BlockEncoder<Code, T>, decoder: BlockDecoder<Code, T> }}
  */
 export const codec = ({ name, code, decode, encode }) => {
   const decoder = new Decoder(name, code, decode)
@@ -19,12 +34,6 @@ export const codec = ({ name, code, decode, encode }) => {
   return { name, code, decode, encode, decoder, encoder }
 }
 
-/**
- * @template {number} Code
- * @template T
- * @typedef {import('./interface').BlockEncoder<Code, T>} BlockEncoder
- */
-
 /**
  * @class
  * @template T
@@ -45,12 +54,6 @@ export class Encoder {
   }
 }
 
-/**
- * @template {number} Code
- * @template T
- * @typedef {import('./interface').BlockDecoder<Code, T>} BlockDecoder
- */
-
 /**
  * @class
  * @template {number} Code

src/codecs/interface.ts

@@ -20,14 +20,11 @@ export interface BlockDecoder<Code extends number, T> {
  * separate those capabilties as sender requires encoder and receiver
  * requires decoder.
  */
-export interface BlockCodec<Code extends number, T> extends BlockEncoder<Code, T>, BlockDecoder<Code, T> {
-  encoder: BlockEncoder<Code, T>,
-  decoder: BlockDecoder<Code, T>
-}
+export interface BlockCodec<Code extends number, T> extends BlockEncoder<Code, T>, BlockDecoder<Code, T> {}
 
 
-// This just a hack to retain type information abouth the data that
-// is incoded `T`  Because it's a union `data` field is never going
+// This just a hack to retain type information about the data that
+// is encoded `T`  Because it's a union `data` field is never going
 // to be usable anyway.
 export type ByteView<T> =
   | Uint8Array