[plt-dev] Some additions

I have added these things, which are available from scheme/base:

* `hash-has-key?' -- a predicate to test if a key is present in a hash
  table.

* `hash-ref!' -- similar to `hash-ref' where the failure argument is
  required, and if the key is not found then this failure argument
  determines the result but also stores it in the table.  (This is the
  function version of the `with-hash' macro that I suggested.)

* `in-sequences' -- a new construct for `for' loops, which
  concatenates sequences.

* `in-cycle' -- similar to `in-sequences', but the sequences are
  repeated in an infinite loop.  Note that unlike Sam's version this
  does not cache the values -- it looks to me like a dangerous
  implicit behavior when you can achieve it explicitly by collecting
  the list first explicitly.  For example, to cycle a file's contents
  you can read it first with `file->bytes'.

-- 
          ((lambda (x) (x x)) (lambda (x) (x x)))          Eli Barzilay:
                  http://www.barzilay.org/                 Maze is Life!