(reverse "") signals an error

[Raymond Toy]

As noted in ​the email from Matt Kaufmann, (reverse "") causes an error.

[toy.raymond@…]

commit 78cce51df441b220c071024fb5e616f1928184dd Author: Raymond Toy <toy.raymond@…> Date: Sat May 18 17:44:02 2013 -0700

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.