Update HTMLBars helpers to use their prototype better.

As requested by @stefanpenner in https://github.com/emberjs/ember.js/pull/9638.

Please note, I could not make the `HandlebarsCompatibleHelper` share the `helperFunction` on the prototype because it is specifically invoked bound to the view context.

I'm open to discussing refactoring away from the view being the context of helpers, but it was specifically requested to be this way by Kris, Mat, and Martin so I'd want to confirm with them before changing away from it.
No it cannot at this time. See the PR description and https://github.com/emberjs/ember.js/pull/9638#discussion_r20595642 for details.