Mask @mod(this) for constructors

[lrytz]

Constructors (almost) always have a @mod(this) effect, but since this is known to be fresh (unless there are some other effects), that could be masked:

@pure class C(x: Int) // does not compile

If there are other effects, the @mod(this) needs to be kept:

class C(@local a: A)

def foo() = {
  vla a = new A
  val c = new C(a) // should have effect @mod(a, c)
}