regexp already merges adjacent overlapping ranges.  this patch
also merges adjacent, non-overlapping ranges..  for example [ab] -> [a-b].
this patch is built on the changes in regexpclass.  the actual change
is 2 lines plus commented-out debugging.

this patch is a significant performance win for some cases:
/tmp/sed is the current sources version.

; time /tmp/sed -n '/[α-ω]/p' </tmp/gs>/dev/null
1.10u 0.02s 1.17r 	 /tmp/sed -n /[α-ω]/p
; time sed -n '/[α-ω]/p' </tmp/gs>/dev/null
1.09u 0.02s 1.17r 	 sed -n /[α-ω]/p

; time /tmp/sed -n '/[αβγδεζηθικλμνξοπρςστυφχψω]/p' </tmp/gs>/dev/null
1.72u 0.02s 1.81r 	 /tmp/sed -n /[αβγδεζηθικλμνξοπρςστυφχψω]/p
; time /bin/sed -n '/[αβγδεζηθικλμνξοπρςστυφχψω]/p' </tmp/gs>/dev/null
1.10u 0.02s 1.17r 	 /bin/sed -n /[αβγδεζηθικλμνξοπρςστυφχψω]/p

debugging output (in the code, commented out)
# before change
; echo x | 8.xsed -n '/[abcdefghijklmnopqrstuvwxyz]/p' 
nspan = 26
a a	0061 0061
b b	0062 0062
c c	0063 0063
d d	0064 0064
e e	0065 0065
f f	0066 0066
g g	0067 0067
h h	0068 0068
i i	0069 0069
j j	006a 006a
k k	006b 006b
l l	006c 006c
m m	006d 006d
n n	006e 006e
o o	006f 006f
p p	0070 0070
q q	0071 0071
r r	0072 0072
s s	0073 0073
t t	0074 0074
u u	0075 0075
v v	0076 0076
w w	0077 0077
x x	0078 0078
y y	0079 0079
z z	007a 007a
x
# after change
; ; echo x | 8.xsed -n '/[abcdefghijklmnopqrstuvwxyz]/p' 
nspan = 1
a z	0061 007a
x