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/inheritance/inherit_fields.wren

@@ -22,7 +22,7 @@ class Bar is Foo {
   }
 }
 
-var bar = new Bar
+var bar = Bar.new()
 bar.foo("foo 1", "foo 2")
 bar.bar("bar 1", "bar 2")