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.

 Evaluate the default-form only if we actually use it.

  Multiple evaluation of the function argument, oops.

  Clozure CL at least considers it currently unused and
  signals a style warning.

  Plus tests.

  No need to depend on the vagaries of ADJUST-ARRAY.

  CIRCULAR-TREE-P had an unfortunate corner case, causing it to overflow
  the stack (seen and repoted at #lisp for '#1=(#1#).)

  The problem is caused by the end-test (of the outer DO) being run before the
  body has a first chance to check for (member slow seen).

 Check for empty sequences and invalid bounding index designators.

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

 Should have done this ages ago.

 Patch by Gustavo on alexandria-devel.

 Also add tests.

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

* Alter `format-symbol` to accept a string designator as the format
  control.
* Use uninterned symbols as format control for type names in type.lisp
  to get the correct names in case-sensitive lisps.

 Make sure we build the documentation from the in-tree version
 of Alexandria.

 * Default doc/Makefile command now generates all docs and cleans up
   temporary files.

 * Generate info as well, and use makeinfo to generate the html.

 * Spit out a ifnottex.include for @&key macros, etc for the benefit
   of makeinfo.

 * Clean up &WHOLE and &ENVIRONMENT from the lambda-lists.

 * MAKE-CIRCULAR-LIST was documented twice, oops.

  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.)

  Easier to work with like this. Note: the code has already mutated a bit.

Patch by: Peter Graves <peter@armedbear.org>

 Call RANDOM just once, and build an inline binary search tree
 for O(log n) goodness instead of O(n).

 Patch by: Gustavo <gugamilare@gmail.com>

 SBCL makes a directory called doc/manual/docstrings; this interferes
 with loading the lisp file if the SBCL source tree had documentation
 built in it unless we use an explicit .lisp suffix.

 Patch by: Daniel Herring <dherring@at.tentpost.dot.com>