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/variable/local_in_middle_of_block.wren

@@ -11,4 +11,4 @@ class Foo {
   }
 }
 
-(new Foo).bar
+Foo.new().bar