MINOR: ist: add ist_find_outside() to find offending characters

MINOR: ist: add ist_find_outside() to find offending characters

For some syntax checks it's interesting to find offending characters
that are outside an interval. Unfortunately for now we only limit
outselves to searching control chars < 0x20 but that's not always
sufficient. Let's just implement a fast scan for characters outside a
range (min and max may not be more than 0x7f apart though). Just like
ist_find_range(), it reads one word at a time and finishes per byte
when found or on false positives. It's optimized for non-matching
inputs.