Make some changes in preparation for integration of SRFI-1 support:

Make some changes in preparation for integration of SRFI-1 support:

. Some of the (simpler) new procedures have been installed verbatim
  from John Kraemer's edit of Olin's implementation.  Some others were
  rewritten, but many others have yet to be installed.

. The mapping procedures have been extended to accept arguments of
  different lengths (as required).

. MEMBER and ASSOC have been extended to take an extra optional
  argument (as required).

. REDUCE has been changed to have the SRFI-1 semantics.  This means
  that (REDUCE CONS '() '(A B C)) now returns

      (c b . a)

  where it used to return

      ((a . b) . c)

  This is an incompatible change; hopefully it won't be too painful
  for the users.

. FOLD-RIGHT has been extended to support multiple list arguments.

. FOLD-LEFT remains unchanged but is now considered obsolete; SRFI-1
  provides FOLD instead, with different semantics.

Additionally, the definition of WEAK-LIST? was wrong and has been
fixed.