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/benchmark/fibers.wren

@@ -5,7 +5,7 @@ var sum = 0
 var start = IO.clock
 
 for (i in 0...100000) {
-  fibers.add(new Fiber {
+  fibers.add(Fiber.new {
     sum = sum + i
     if (i < 99999) fibers[i + 1].call()
   })