Make constructors just methods.

* Eliminate "new" reserved word.
* Allow "this" before a method definition to define a constructor.
* Only create a default constructor for classes that don't define one.

test/language/setter/prefix_operator.wren

@@ -2,5 +2,5 @@ class Foo {
   bar=(value) { value }
 }
 
-var foo = new Foo
+var foo = Foo.new()
 !foo.bar = "value" // expect error