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/method/long_name.wren

@@ -4,4 +4,4 @@ class Foo {
   }
 }
 
-IO.print((new Foo).thisHasAMethodNameThatIsExactly64CharactersLongWhichIsTheMaximum) // expect: result
+IO.print(Foo.new().thisHasAMethodNameThatIsExactly64CharactersLongWhichIsTheMaximum) // expect: result