Add support for WeakMap, WeakSet and Promise

[fabiospampinato]

As an example the following returns true, instead it should return false:

require ( 'fast-deep-equal/es6' )( new WeakMap (), new WeakMap () )

I think this types of objects should be checked for strict equality.

[illuminist]

I believe WeakMap and WeakSet don't hold or have an ability to iterate their keys. So they just can't be compared.

Had no idea about Promise equality though.

[fabiospampinato]

@illuminist they can't be deeply compared like objects, but you can check if they are the same object or not.

[illuminist]

Oh I see, I misread your explanation. Good day though.

[epoberezkin]

@fabiospampinato That is a breaking change, but it makes sense to not just iterate their properties and only compare by reference. PR is welcome.