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/inheritance/inherit_methods.wren

@@ -12,7 +12,7 @@ class Bar is Foo {
   override { IO.print("bar") }
 }
 
-var bar = new Bar
+var bar = Bar.new()
 bar.methodOnFoo // expect: foo
 bar.methodOnBar // expect: bar