remove `JsFunction` type parameter

[dherman]

From the list of remaining breaking changes, this PR removes the JsFunction type parameter. This is safe for the following reasons:

• Neon functions are non-strict, which means when they are invoked with a non-object (e.g. with f.call(null) or f.call(17)) the primitive is replaced by an object.
• The JsObject type does not assume the object inherits from Object.prototype, so it works for any JS objects, even exotic objects like Object.create(null), proxies, module objects, etc.

[dherman]

I added a unit test that uses the global object, so it'll have a small merge conflict with #987.

[kjvalencik]

I love the tests checking that constructions are still objects! ❤️