fix: allow CIDs to be compared through deep equality

The changes in #131 broke the tests of IPFS and probably quite a few
other modules.

This sort of thing used to work, now does not:

```js
expect(ipfs.bitswap.unwant.calledWith(new CID(cidStr), defaultOptions)).to.be.true()
```

The reason it breaks is because internally `calledWith` does a `deepEqual`
on the args which compares (among other things) the properties of the passed
objects.

We used to use `Object.defineProperty` to create cached versions of
expensive to calculate fields which makes fields non-enumerable by
default so they are skipped during the `deepEqual` check.

different fields depending on how which constructor branch was hit or
worse, if the instances properties have been accessed.

package.json

@@ -45,6 +45,7 @@
     "uint8arrays": "^1.1.0"
   },
   "devDependencies": {
+    "@sinonjs/samsam": "^5.3.0",
     "aegir": "^29.0.1",
     "multihashing-async": "^2.0.1"
   },

src/index.js

@@ -95,7 +95,7 @@ class CID {
         this.multibaseName = 'base58btc'
       }
       CID.validateCID(this)
-      this.string = version
+      Object.defineProperty(this, 'string', { value: version })
       return
     }
 
@@ -164,6 +164,7 @@ class CID {
    *
    */
   get bytes () {
+    // @ts-ignore
     let bytes = this._bytes
 
     if (!bytes) {
@@ -179,7 +180,7 @@ class CID {
       }
 
       // Cache this Uint8Array so it doesn't have to be recreated
-      this._bytes = bytes
+      Object.defineProperty(this, '_bytes', { value: bytes })
     }
 
     return bytes
@@ -248,7 +249,9 @@ class CID {
    * @returns {string}
    */
   toBaseEncodedString (base = this.multibaseName) {
+    // @ts-ignore non enumerable cache property
     if (this.string && this.string.length !== 0 && base === this.multibaseName) {
+      // @ts-ignore non enumerable cache property
       return this.string
     }
     let str
@@ -264,7 +267,7 @@ class CID {
     }
     if (base === this.multibaseName) {
       // cache the string value
-      this.string = str
+      Object.defineProperty(this, 'string', { value: str })
     }
     return str
   }

test/index.spec.js

@@ -8,6 +8,7 @@ const multihashing = require('multihashing-async')
 const uint8ArrayFromString = require('uint8arrays/from-string')
 const uint8ArrayToString = require('uint8arrays/to-string')
 const CID = require('../src')
+const { deepEqual } = require('@sinonjs/samsam')
 
 describe('CID', () => {
   let hash
@@ -448,4 +449,47 @@ describe('CID', () => {
       expect(cid.toBaseEncodedString()).to.equal(base32String)
     })
   })
+
+  describe('equality', () => {
+    it('should be deeply equal', () => {
+      const cid1 = new CID('QmdfTbBqBPQ7VNxZEYEj14VmRuZBkqFbiwReogJgS1zR1n')
+      const cid2 = new CID('QmdfTbBqBPQ7VNxZEYEj14VmRuZBkqFbiwReogJgS1zR1n')
+
+      expect(deepEqual(cid1, cid2)).to.be.true()
+    })
+
+    it('should be deeply equal when constructed from another CID', () => {
+      const cid1 = new CID('QmdfTbBqBPQ7VNxZEYEj14VmRuZBkqFbiwReogJgS1zR1n')
+      const cid2 = new CID(cid1)
+
+      expect(deepEqual(cid1, cid2)).to.be.true()
+    })
+
+    it('should be deeply equal when constructed from an Uint8Array', () => {
+      const cid1 = new CID('QmdfTbBqBPQ7VNxZEYEj14VmRuZBkqFbiwReogJgS1zR1n')
+      const cid2 = new CID(cid1.multihash)
+
+      expect(deepEqual(cid1, cid2)).to.be.true()
+    })
+
+    it('should still be deeply equal after turning one into a base encoded string', () => {
+      const cid1 = new CID('QmdfTbBqBPQ7VNxZEYEj14VmRuZBkqFbiwReogJgS1zR1n')
+      const cid2 = new CID(cid1.multihash)
+      const cid3 = new CID(cid1.multihash)
+
+      cid3.toBaseEncodedString()
+
+      expect(deepEqual(cid2, cid3)).to.be.true()
+    })
+
+    it('should still be deeply equal after turning one into bytes', () => {
+      const cid1 = new CID('QmdfTbBqBPQ7VNxZEYEj14VmRuZBkqFbiwReogJgS1zR1n')
+      const cid2 = new CID(cid1.multihash)
+      const cid3 = new CID(cid1.multihash)
+
+      cid3.bytes // eslint-disable-line no-unused-expressions
+
+      expect(deepEqual(cid2, cid3)).to.be.true()
+    })
+  })
 })