Memoization mods to fix performance monitoring and perhaps increase efficiency slightly.

Also, tweaked :doc git-quick-start.  (In particular, paragraph
end/start markers seemed to be missing but I liked how the first two
"paragraphs" were shown as a single paragraph, so I eliminated the
blank line between them.)

When a function is memoized with lower-overhead or :stats nil, the
performance monitoring statistics (for (memsum)) no longer consider
that function.  The statistics previously were, I think, a bit messed
up.  If you evaluate the following sequence of forms before and after
this commit, you'll see what I mean.

(defn f1 (x) x)
(defn f2 (x) (f1 x))
(defn f3 (x) (f2 x))
(memoize 'f1)
(memoize 'f2 :stats nil)
(memoize 'f3)
(progn$ (f3 3) (f3 3) (f3 3) (f3 3) (f3 10))
(memsum)

I also tweaked code generation for memoized functions to improve
efficiency, though it's not clear that it made much of a difference.
Here are some test results.

; Original version:

; (EV-REC *RETURN-LAST-ARG3* ...) took
; 96.65 seconds realtime, 96.49 seconds runtime
; (4,167,707,904 bytes allocated).

; New version:

; 96.39 seconds realtime, 96.16 seconds runtime
; (4,167,707,392 bytes allocated).

; Original version, starting with (lower-overhead):

; 96.19 seconds realtime, 96.02 seconds runtime
; (4,167,707,904 bytes allocated).

; New version, starting with (lower-overhead):

; 95.68 seconds realtime, 95.46 seconds runtime
; (4,167,707,904 bytes allocated).

Technical changes:

- In memoized function code generation, a call of
  unwind-protect-disable-interrupts-during-cleanup has been replaced
  by a prog1, multiple-value-prog1, or progn, which might slightly
  increase efficiency with the trade-off that performance monitoring
  statistics could be off a bit when there is an interrupt.

- To support the change above, *caller* and the start-ticks variables
  are ordinary specials, no longer introduced with defg.

- In memoized function code generation, FLET is no longer used, which
  might improve efficiency slightly but could cause code size to
  double or, if a :condition is supplied, triple.

- Memoized function code generation has an optimization when
  :condition is t or 't, which is now significant since it omits
  laying down one call of the body now that FLET is no longer used.

- Memoized function code is a bit more efficient when :aokp is true.
  More important probably, the code for memoize-fn-inner-body is a bit
  more readable.

- A missing mfixnum declaration was added for the start-ticks variable
  in memoize-fn-outer-body.