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/nonlocal/mutual_recursion.wren

@@ -1,9 +1,9 @@
 class Foo {
-  static bar { new Bar }
+  static bar { Bar.new() }
 }
 
 class Bar {
-  static foo { new Foo }
+  static foo { Foo.new() }
 }
 
 IO.print(Foo.bar) // expect: instance of Bar