Added Codewalker as requested by J Moore.  Also, updated some documentation.

Quoting :doc note-7-0-books:

  The new projects/codewalker/ directory contains Codewalker, a utility
  for exploring code in any programming language specified by an ACL2
  model to discover certain properties of the code. Demos of
  Codewalker are also in that directory.

Added this to :doc note-7-0 for preceding commit/push:

  We made a low-level implementation tweak that resulted in a speed-up
  of 2.6% in wall-clock time for a CCL-based regression (using
  ``make'' with -j 8 and target ``everything''). (Technical note: We
  eliminated the Lisp special variable *attached-fn-called*, using
  another variable *aokp* instead. Special variable bindings can
  apparently be expensive!)

Also, fixed recent mod to :doc coerce, thanks in part to feedback from
J.