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.

  Plus tests.

 Patch by Gustavo on alexandria-devel.

 Also add tests.

  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.)

Type declareations like ((or fixnum null) bar) or (unsigned-byte foo)
don't work on CLISP. Must use (type unsigned-byte foo) instead.