Use __proto__ for fn extensions on browsers that support it.
Based on work by @mwiencek and @alFReD-NSH
Specs for "fn" in general
Ensure spec count remains consistent across browsers

Otherwise it’s hard to tell if some syntax error has caused a bunch of
specs not to run on a particular browser
Added equivalent proto assignment spec for ko.subscribable
Instead of overwriting ko.*.fn.__proto__ every time an observable/subscribable/observableArray is constructed, define the inheritance chain in a one-time setup action
Make sure observables can use Function.prototype methods when overwriting __proto__
Merge branch '775-proto-for-observables'