Add isDeepEqual function

[aleclarson]

Reference implementation: https://github.com/lukeed/dequal/blob/master/src/index.js

Requested by #226

Notes

• Not sure if ArrayView or DataView support is necessary.

• Might be good to have an option for customization of object handling (though arrays, Map instances, and Set instances would not be customizable). For example, Lodash's isEqual has special handling for DOM nodes that some people may want. It would also let us keep the implementation lean (e.g. for those who don't use ArrayView and DataView).

  This could be through an extend static method.

  // The extend callback is only called when two objects with the same constructor
  // are compared. This does not include arrays, maps, or sets.
  const myDeepEqual = isDeepEqual.extend((left, right) => {
    // return true if left and right are equal
  })

  …or a direct option (more cumbersome IMO):

  if (isDeepEqual(foo, bar, (left, right) => {
    // return true if left and right are equal
  })) {
    console.log('foo and bar are equal')
  }