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/this/this_in_method.wren

@@ -3,4 +3,4 @@ class Foo {
   baz { "baz" }
 }
 
-IO.print((new Foo).bar.baz) // expect: baz
+IO.print(Foo.new().bar.baz) // expect: baz