[R6RS] record-constructor suggestion

[dyb at cs.indiana.edu]

I suggest that we add an rtd argument to record-constructor and make the
record-constructor-descriptor argument optional.  This simplifies the
common case (reflected in the two uses of record-constructor in the
examples) where a default constructor is desired from:

  (record-constructor
    (make-record-constructor-descriptor rtd
      #f #f))

to

  (record-constructor rtd)

Any objections?

Kent