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/closure/close_over_later_variable.wren

@@ -3,10 +3,10 @@
 // would crash because it walked to the end of the upvalue list (correct), but
 // then didn't handle not finding the variable.
 
-new Fn {
+Fn.new {
   var a = "a"
   var b = "b"
-  new Fn {
+  Fn.new {
     IO.print(b) // expect: b
     IO.print(a) // expect: a
   }.call()