Update to v6.11

* Typed Racket supports refinement types and dependent function types.
  Previously an experimental feature, refinement types allow types to
  describe more interesting properties of values, especially integers.
  For example, this type shows that the `max` function always produces a
  number at least as big as its inputs:
  `(-> ([x : Integer] [y : Integer])
       (Refine [z : Integer] (and (>= z x) (>= z y))))`

* DrRacket's Program Contour is significantly more efficient; using it
  no longer hurts DrRacket's interactivity.

* The `web-server/formlets` library produces formlets that are
  serializable, facilitating dynamic uses of formlets in stateless #lang
  web-server servlets. The new `web-server/formlets/stateless` and
  `web-server/formlets/unsafe` libraries provide additional support with
  the same API.

* The `db` library supports the Cassandra database.


(sevan)