feat!: CID as an interface

This is an attempt at implementing #203, converting the CID class
into a minimal interface and having factory functions to return
objects that conform to the CID interface.

I've preserved all the generics the `Link` interface introduced and
there are functions in `link.js` that add the extra methods to turn
something that conforms to `CID` into something that can be used as
a `Link` so existing code using the `Link` API should not have to
change.

Notably there was no need to update any of the `Link` tests.

The static methods on CID have been exported as individual functions
- the names remain the same (`decode`, `parse`, etc) in an attempt
to be less disruptive.

Code using these methods should mostly just need to change:

```js
import { CID } from 'multiformats/cid'
```

to:

```js
import * as CID from 'multiformats/cid
```

Types can be imported as:

```ts
import type { CID } from 'multiformats/interface'
```

or as before:

```ts
import type { CID } from 'multiformats/cid'
```

BREAKING CHANGE: the CID class is now an interface

Author: achingbrain

README.md

@@ -36,7 +36,7 @@ This library defines common interfaces and low level building blocks for various
 This library provides implementations for most basics and many others can be found in linked repositories.
 
 ```js
-import { CID } from 'multiformats/cid'
+import * as CID from 'multiformats/cid'
 import * as json from 'multiformats/codecs/json'
 import { sha256 } from 'multiformats/hashes/sha2'
 

src/block.js

@@ -2,6 +2,13 @@ import { bytes as binary, CID } from './index.js'
 // Linter can see that API is used in types.
 // eslint-disable-next-line
 import * as API from './interface.js'
+// Linter can see that API is used in types.
+// eslint-disable-next-line
+import * as CIDAPI from './cid/interface.js'
+// Linter can see that API is used in types.
+// eslint-disable-next-line
+import * as LinkAPI from './link/interface.js'
+import { asLink } from './link.js'
 
 function readonly ({ enumerable = true, configurable = false } = {}) {
   return { enumerable, configurable, writable: false }
@@ -10,7 +17,7 @@ function readonly ({ enumerable = true, configurable = false } = {}) {
 /**
  * @param {[string|number, string]} path
  * @param {any} value
- * @returns {Iterable<[string, CID]>}
+ * @returns {Iterable<[string, CIDAPI.CID]>}
  */
 function * linksWithin (path, value) {
   if (value != null && typeof value === 'object') {
@@ -39,7 +46,7 @@ function * linksWithin (path, value) {
  * @template T
  * @param {T} source
  * @param {Array<string|number>} base
- * @returns {Iterable<[string, CID]>}
+ * @returns {Iterable<[string, CIDAPI.CID]>}
  */
 function * links (source, base) {
   if (source == null || source instanceof Uint8Array) {
@@ -121,7 +128,7 @@ function get (source, path) {
 class Block {
   /**
    * @param {object} options
-   * @param {CID<T, C, A, V>} options.cid
+   * @param {LinkAPI.Link<T, C, A, V>} options.cid
    * @param {API.ByteView<T>} options.bytes
    * @param {T} options.value
    */
@@ -176,14 +183,14 @@ async function encode ({ value, codec, hasher }) {
 
   const bytes = codec.encode(value)
   const hash = await hasher.digest(bytes)
-  /** @type {CID<T, Code, Alg, 1>} */
+  /** @type {CIDAPI.CID<T, Code, Alg, 1>} */
   const cid = CID.create(
     1,
     codec.code,
     hash
   )
 
-  return new Block({ value, bytes, cid })
+  return new Block({ value, bytes, cid: asLink(cid) })
 }
 
 /**
@@ -194,18 +201,18 @@ async function encode ({ value, codec, hasher }) {
  * @param {API.ByteView<T>} options.bytes
  * @param {API.BlockDecoder<Code, T>} options.codec
  * @param {API.MultihashHasher<Alg>} options.hasher
- * @returns {Promise<API.BlockView<T, Code, Alg>>}
+ * @returns {Promise<API.BlockView<T, Code, Alg, 1>>}
  */
 async function decode ({ bytes, codec, hasher }) {
   if (!bytes) throw new Error('Missing required argument "bytes"')
   if (!codec || !hasher) throw new Error('Missing required argument: codec or hasher')
 
   const value = codec.decode(bytes)
   const hash = await hasher.digest(bytes)
-  /** @type {CID<T, Code, Alg, 1>} */
+  /** @type {CIDAPI.CID<T, Code, Alg, 1>} */
   const cid = CID.create(1, codec.code, hash)
 
-  return new Block({ value, bytes, cid })
+  return new Block({ value, bytes, cid: asLink(cid) })
 }
 
 /**
@@ -229,8 +236,7 @@ function createUnsafe ({ bytes, cid, value: maybeValue, codec }) {
   if (value === undefined) throw new Error('Missing required argument, must either provide "value" or "codec"')
 
   return new Block({
-    // eslint-disable-next-line object-shorthand
-    cid: /** @type {CID<T, Code, Alg, V>} */ (cid),
+    cid,
     bytes,
     value
   })

src/block/interface.ts

@@ -1,7 +1,7 @@
 /* eslint-disable @typescript-eslint/no-unnecessary-type-constraint */
 /* eslint-disable no-use-before-define */
 import { Link, Version } from '../link/interface.js'
-import { CID } from '../cid.js'
+import { CID } from '../cid/interface.js'
 
 /**
  * A byte-encoded representation of some type of `Data`.
@@ -64,7 +64,7 @@ export interface BlockView<
   A extends number = number,
   V extends Version = 1
 > extends Block<T, C, A, V> {
-  cid: CID<T, C, A, V>
+  cid: Link<T, C, A, V>
   value: T
 
   links: () => Iterable<[string, CID]>