td.constructor() feature for creating fake constructors

[searls]

This is already basically implemented to support td.replace() (source) of constructors, but it's not exposed in an explicit way.

Given a constructor:

function Dog(){}
Dog.prototype.bark = function(){}
Dog.prototype.woof = function(){}

One could call (in environments supporting destructured assignment):

var [FakeDog, dog] = td.constructor(Dog)

Where FakeDog is a constructor that can be passed to the subject or otherwise replaced and dog is a plain JavaScript object of the same set of test doubles one would get if they instantiated new FakeDog().

One point of potential confusion for users is that instantiating FakeDog multiple times will yield the same set of fake functions, so the total number of stubbings & invocations would accumulate over all FakeDog instances, which would run counter to our expectations of statefulness with instantiated objects, but would be necessary for the test to have sane access to the test double functions. Feedback on this point would be welcome.

[searls]

Reminder in #164 that this should work on ES6 class static methods

[searls]

Closing in favor of #166, in which case td.replace will give you exactly what td.constructor would have.