Cleaned up memoization of built-ins at startup; new memoize keyword :stats; memoization stays on in ACL2(hp) even with waterfall-parallelism on.

Dealt with the following comment from memoize-raw.lisp (and thus
deleted it); see first item from :doc note-7-0 below.

; - Maybe think a bit harder about whether our initial memoizations (calls of
;   memoize-fn in acl2h-init-memoizations) are really handled properly, given
;   that our code is so oriented towards calling memoize in the loop.  The
;   current approach seems to have worked well for a long time, so maybe this
;   isn't much of a priority.  But it seems a bit dodgy somehow.

Quoting :doc note-7-0:

    * For every built-in memoized function defined in the ACL2 logic, the
      memoization is done in the usual way (during the ACL2 build),
      using the [memoize] event during the ACL2 build. Such functions
      can thus be [unmemoize]d by users in the usual way. Thanks to
      David Rager for requesting this enhancement.
    * A new memoize keyword, :stats, can control whether statistics will be
      saved for reporting with (memsum).
    * Memoization in ACL2(hp) is no longer affected by calls of
      [set-waterfall-parallelism].