fix issue #2640: support instanceof operator

[alburthoffman]

refer to #2640, add code to support instanceof operator and make tests.

[mattjohnsonpint]

I can see that this will work, but I'm wondering if there could be unforeseen side effects. The current moment.prototype points at the utils_hooks__hooks function. This seems rather deliberate.

@ichernev @timrwood

[ichernev]

@mj1856 you're wrong. The current moment.prototype is the default prototype for all created objects -- an empty object (I verified that with a simple test). So this should be safe to merge.

[ichernev]

Merged in aee4e2f