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/fiber/closure.wren

@@ -3,7 +3,7 @@ var closure
 
 {
   var a = "before"
-  fiber = new Fiber {
+  fiber = Fiber.new {
     IO.print(a)
     Fiber.yield()
     a = "after"
@@ -12,7 +12,7 @@ var closure
     a = "final"
   }
 
-  closure = new Fn {
+  closure = Fn.new {
     IO.print(a)
   }
 }