Classes: make private fields and methods class-private, not instance-private

[catamorphism]

And add compile-fail tests to enforce this.

[graydon]

To clarify, this means that two instances of the same class should be able to call one another's private methods?

[catamorphism]

@graydon Yes. I actually am not personally in favor of this, but it is what @pcwalton 's original proposal said so I figured I ought to stick to it for now.

[brson]

Obsolete with max/min classes.