Prevent polluting prototypes with enumerable props

[JGreenlee]

Fixes #816

Assigning functions directly to the prototypes of Number, Array, String, etc., causes them to be enumerable, meaning they can show up in "for .. in .." loops and cause unexpected behavior!

Instead we can use Object.defineProperty (for which there is already a helper called __setproperty__). With this method, i) new props are non-enumerable by default, and ii) we can avoid unnecessarily reassigning the same methods (in case there are multiple instances of Transcrypt running).

[JGreenlee]

Put dict methods on the dict.prototype

I noticed that list methods get assigned to Array.prototype, str methods get assigned to String.prototype, etc.
dict methods do not get assigned to Object.prototype (I am guessing this is for a good reason)

The current code has the dict methods being added to each new instance in the constructor; but it is tidier to declare them on the dict prototype and then create each dict instance from that.
This is also more efficient because properties placed on the dict prototype need only be declared once, as opposed to being declared for each dict.

[JGreenlee]

The properties & prototype chain for an example dict foo:

Before (on master):

After changes

[JennaSys]

This is one I've had an eye on to tackle at some point sooner than later, so thanks for looking into it. I can't dig into it right at this moment, but are there any possible unintended side effects by changing this that you can think of?

[JGreenlee]

If for some reason, someone was expecting the methods of str or list to be included while iterating over an Array or String, they would no longer see those.

for (let e in []) {
  console.log(e)
}
// on master, prints all the methods of py `list`
// after this PR, prints nothing

I don't know why someone would be relying on that behavior though.