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/variable/local_collide_with_function_parameter.wren

@@ -1,3 +1,3 @@
-new Fn {|a|
+Fn.new {|a|
   var a = "oops" // expect error
 }