Add `items`, `values` and `keys` methods to HyDict

Useful for macros, for example:

    (defmacro defn-checked [n x b]
      `(defn ~n [(unquote-splice (.keys x))]
        (print "Expecting:" (unquote-splice (.values x))) ~b))

    (defn-checked hello {name str age int}
      (print "Hello" name "you are" age))

    (hello "Godzilla" 60)

    ;; Expecting: <type 'str'> <type 'int'>
    ;; Hello Godzilla you are 60