Fix ticket:81 and fix ticket:83.

From ticket 81, the tests are now:

{{{
(time (prog1 t (time-rev *s*)))
; Evaluation took:
;   0.49 seconds of real time
;   0.481813 seconds of user run time
;   0.003624 seconds of system run time
;   1,490,776,936 CPU cycles
;   [Run times include 0.13 seconds GC run time]
;   0 page faults and
;   200,073,704 bytes consed.

(time (prog1 t (time-rev *s2*)))
; Evaluation took:
;   0.97 seconds of real time
;   0.965893 seconds of user run time
;   0.005139 seconds of system run time
;   2,980,415,911 CPU cycles
;   [Run times include 0.23 seconds GC run time]
;   0 page faults and
;   400,005,560 bytes consed.
}}}

So the new string-reverse* is 20 times faster for strings without
surrogates and 10 times faster for strings containing only surrogates.