Changes to Scala syntax for better effect syntax

[lrytz]

Three syntax changes we'd like to push to Scala:

• Allow return type (and therefore effect) on constructors

class C(x: Int): C @pure extends D {
}

• Same thing for auxiliary constructors

class C extends D {
  def this(x: Int): C @pure = { ... }
}

• Allow result type annotations without result type (meaning: infer type, but check effect)

def foo(x: Int): @pure = x + 1

The first two will eliminate the special cases for constructor effect annotations.