Return NIL if the sequence is empty, instead of the NO-EXTREMUM nonsense.

 It was bad design, because it's not an error someone higher up the stack can
 sensibly handle, and handling it locally is too verbose and slow.

    (or (extremum ...) (error ...))

 expresses the common case succintly, and fits the pattern of existing
 sequence functions.

 If it is deemed necessary, we can also add &KEY DEFAULT, but that seems
 overkill and has little precedent in sequence functions.

  From http://www.cliki.net/EXTREMUM

  A simple version built on top of REDUCE for now.

  This reverts commit 485544d4.

  MEAN and MEDIAN are again non-generic, and DISPLACE-ARRAY is removed.

  Motivation on the mailing list.

 Objects other than sequences have means and medians (eg probability
 distributions, arrays, sequences/arrays wrapped in another object, etc), so
 it makes sense to make these functions generic.

 DISPLACE-ARRAY is a small utility function that is used in the implementation
 of MEDIAN for arrays, but is also of general utility because it makes the
 creation of displaced arrays simpler, dispensing with the need to specify the
 element type, and offering sensible defaults when one wants vectors.

 Also added tests for all the new functions/methods.

 ...I of all people should know this...

 Should have done this ages ago.

  I've been wondering about adding these for a fair while
  now, but they _are_ nice when writing eg. certain kinds
  of macros,

  Add bunch of missing symbols to the manual, and go over several
  docstrings reformatting them for the benefit of the documentation
  generator.

  (A crapton of symbols are still not in the manual.)

- New function: PROPER-LIST-LENGTH.

- Share code between PROPER-LIST-LENGTH, LASTCAR, and SETF LASTCAR
  via a macrolet.

- Use ENDP wrapped in SAFETY 3, to ensure proper detection of dotted
  lists.

- Use PROPER-LIST-LENGTH instead of LIST-LENGTH in ROTATE, RANDOM-ELT,
  and SHUFFLE, and test that they signal an error for improper lists.
  (Thanks to Tobias Rittweiler)

with-input-from-file
with-output-to-file
read-file-into-string
write-string-into-file
copy-stream
copy-file

	* package.lisp: Export UNWIND-PROTECT-CASE.

 * Like DOLIST, but iterates over plists.

 * Thanks to Andreas Fuchs -- I only took the liberty of changing the name from
   MULTIPLE-VALUE-OR to NTH-VALUE-OR.

 * new function: like GETHASH, but saves the default value in table if
   key is not found.*

- Added respective documentation to the manual.
- New test: NCONCF.1

Added respective documentation in manual as well.

Added respective documentation to the manual.