introduce td.constructor() as top-level API sibling to td.object() & td.function()

[searls]

Fortunately 2.0.0 is in preview, because I just realized this morning how horrible of an impedance mismatch I've created by having td.object() accept constructor functions and returning fake constructors.

How it used to work

This is how we got here

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

var fakeDog = td.object(Dog) // a function bag

fakeDog.bark // <-- a test double function.

td.object() would return a function bag by inspecting the constructor you pass it.

That changed!

And now in 2.0 when you pass a constructor to td.object, you get a fake constructor with test double functions on the prototype.

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

var FakeDog = td.object(Dog) //<-- a constructor

FakeDog.prototype.bark // <-- a test double function.

Changes in this PR:

• you can no longer pass a constructor function to td.object
• new API that is td.constructor that takes a real constructor and returns a fake constructor
• accept similar input to td.object (namely, an array of function names creates a fake constructor from nothing) td.constructor(['bark'])
• improve the toString impl for fake instances
• fix a bunch of tests
• add name to td.explain
• add error to td.object
• fix the repl
• update docs for td.object
• add doc for td.constructor (namely that you can pass a constructor or an array of names
• update doc for td.explain