Add support for BigInt as a primitive

[Jason Dreyzehner]

Thanks for the awesome work on this library! It's been fantastic. Do you have any plans for supporting BigInt?

The following AVA test generates an error in Concordance (Node v10.10.0):

  t.deepEqual(
    {
      a: BigInt(0)
    },
    {
      b: BigInt(0)
    }
  );

Error:

/[project]/node_modules/concordance/lib/Registry.js:20

  Error thrown in test:

  TypeError {
    message: 'Invalid value used as weak map key',
  }

  Registry.alloc (node_modules/concordance/lib/Registry.js:20:14)
  describeComplex (node_modules/concordance/lib/describe.js:102:28)
  curriedComplex (node_modules/concordance/lib/describe.js:141:12)
  Object.describeAny (node_modules/concordance/lib/describe.js:148:9)
  Object.next (node_modules/concordance/lib/complexValues/object.js:107:48)
  Object.next (node_modules/concordance/lib/recursorUtils.js:59:44)
  Object.recursor (node_modules/concordance/lib/complexValues/object.js:185:31)
  compareDescriptors (node_modules/concordance/lib/compare.js:70:32)
  Object.compare (node_modules/concordance/lib/compare.js:100:16)

[Mark Wubben]

Yep let's do this.