Class variable

[BCSharp]

This is another take on #723 (an alternative to #734). As such it partially undoes #721 (parser changes).

It is still a hack, because __class__ variables are now manages as global variables named $__class__NN, not an ideal situation, but it is meant as a proof-of-concept for how a __class__ variable should behave. In particular, it seems that a __class__ variable behaves as a regular class scope variable that just happen to be initialized to the class type. For instance, it is possible to set __class__ to any value, even if it is not a type (e.g. it is possible to set it to a number), while setting the attribute __class__ is guarded against invalid values.

Alternative implementations to global variables:

1. use an ordinary local variable as __class__, placed in the class scope and lifted to be a closure cell; this is my favourite, but I was unable to get it work (2ac9d8c)
2. use a closure from the parent scope to place __class__; it works OK but only for non-top-level classes and still pollutes the namespace, this time the local one
3. use a separate closure from the parent closure, which is managed by the class scope itself; since 2. works, this should be possible as well, but I was unable to get it to work (with the same error as with 1., so maybe I am overlooking something) (2d95476)
4. use a separate dedicated storage that is out of sight, perhaps as part of the context

@slozier @isaiah @in-code-i-trust

[slozier]

Closing this in favor of #1087.