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/sequence/count.wren

@@ -8,4 +8,4 @@ class TestSequence is Sequence {
   iteratorValue(iterator) { iterator }
 }
 
-IO.print((new TestSequence).count) // expect: 10
+IO.print(TestSequence.new().count) // expect: 10