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_from_closure.wren

@@ -2,7 +2,7 @@ var ClosureType
 
 {
   var a = "a"
-  ClosureType = new Fn { IO.print(a) }.type
+  ClosureType = Fn.new { IO.print(a) }.type
 }
 
 class Subclass is ClosureType {} // expect runtime error: Subclass cannot inherit from Fn.