Classes: handle typestate correctly in ctors

[catamorphism]

Enforce these invariants in typestate:

"The constructor must initialize all fields of the object and cannot call any methods on self until it has done so." (currently there's no check that the ctor initializes fields, even)

"After calling a method on self, the constructor is not allowed to mutate any of its immutable fields."

[catamorphism]

Blocked on #2376 -- if that RFC gets accepted, we'll be able to eliminate ctors and there'll be no need for special cases in typestate.

[nikomatsakis]

this is implemented in the new liveness code. if I'd realized it wasn't implemented at all in typestate I probably wouldn't have bothered, but oh well. =)