This is similar to ec69b4bb1256c061ac76f53dfed09c4283ec6a31, which
was about private instance fields.
Private properties can be non-writable (thanks to decorators), or have
get/set accessors. If we stored this information on the `privateClass`
object, we would need to always use `Object.getOwnPropertyDescriptor`
before reading or writing a property because accessors need to be called
with the correct `this` context (it should be the actual class, not the
object hat stores the private properties). This commit simplifies that
operation a bit by removing the container object.
It also have another advantage, which instance fields already have
thanks to the use of separate weakmaps: unused private static fields
can be tree-shaken away or garbage-collected, while properties of an
object can't. Also, they can be easilier minified.
* Private Static Class Fields Implementation
* Redo testing without class transform for static private
* Add a call() test for private static
* Use Object.defineProperty in loose mode
