Cache parsed range patterns

Measurement:

(script)
use utf8;
my $generator = String::Random->new;
$generator->randregex('[一-龠]{5}') for 1..1000;

(time)
before: 3.63s
after:  1.54s

(CPU environment)
Intel(R) Core(TM) i5-4670 CPU @ 3.40GHz