Use more conventional syntax for constructors.

They are now invoked like "new Foo".
Also, superclass constructors are now much less semantically
and syntactically weird. Since the instance is created before
any constructor is called, there's no point in time where the
instance isn't there.

test/constructor/new.wren

@@ -1,18 +1,17 @@
 class Foo {
-  // TODO: Do we want to require an explicit "new" here?
-  this new { io.write("zero") }
-  this new(a) { io.write(a) }
-  this new(a, b) { io.write(a + b) }
+  new { io.write("zero") }
+  new(a) { io.write(a) }
+  new(a, b) { io.write(a + b) }
 
   toString { return "Foo" }
 }
 
 // Can overload by arity.
-Foo.new // expect: zero
-Foo.new("one") // expect: one
-Foo.new("one", "two") // expect: onetwo
+new Foo // expect: zero
+new Foo("one") // expect: one
+new Foo("one", "two") // expect: onetwo
 
 // Returns the new instance.
-var foo = Foo.new // expect: zero
+var foo = new Foo // expect: zero
 io.write(foo is Foo) // expect: true
 io.write(foo.toString) // expect: Foo