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/core/fiber/resume_caller.wren

@@ -1,8 +1,8 @@
-var b = new Fiber {
+var b = Fiber.new {
   IO.print("fiber b")
 }
 
-var a = new Fiber {
+var a = Fiber.new {
   IO.print("begin fiber a")
   b.call()
   IO.print("end fiber a")