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.

Now we just set the fields on the object which means instances have
different fields depending on which constructor branch was hit or worse,
if the instances properties have been accessed.

Author: achingbrain

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()
+    })
+  })
 })