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/constructor/cannot_be_subscript.wren

@@ -0,0 +1,5 @@
+class Foo {
+  this [value] { // expect error
+    IO.print("ok")
+  }
+}