MINOR: ist: add new function ist_find_range() to find a character range

MINOR: ist: add new function ist_find_range() to find a character range

This looks up the character range <min>..<max> in the input string and
returns a pointer to the first one found. It's essentially the equivalent
of ist_find_ctl() in that it searches by 32 or 64 bits at once, but deals
with a range.