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/is_empty.wren

@@ -7,4 +7,4 @@ class InfiniteSequence is Sequence {
 }
 
 // Should not try to iterate the whole sequence.
-IO.print((new InfiniteSequence).isEmpty) // expect: false
+IO.print(InfiniteSequence.new().isEmpty) // expect: false