Classes: add tests for allocating classes in various regions

[catamorphism]

(that is, make sure that @-classes work and that classes can be allocated on the exchange heap. Right now I think classes are either always or never stack-allocated. I need to sort this out.)

[catamorphism]

(to fix this, I will probably have to change the translation for ctors to allocate memory for the self object outside the constructor, and pass it into the ctor code)

[catamorphism]

Deferring this after discussion with @nikomatsakis and @pcwalton . We agreed that @-classes probably aren't useful enough to be worth the effort. Instead, I'll file an RFC for static fns, which would address some of the same problems.

[catamorphism]

(Closing this issue is blocked on the outcome of #2376)

[brson]

Obsolete?

[catamorphism]

Yes, obsolete; this is one of the things max/min classes explicitly addresses.