The previous strategy wasn't robust enough to detect typos in function
calls. For example, if you misremembered String.duplicate/2 as
String.repeat/2 the koan runner would hang and have to be restarted.
This is because the stacktrace for an error like that doesn't have line
numbers. To avoid the problem, we detect the first pieces of the
stracktrace that is in the koan module. I can't decide if that's a
perfect solution, but it seemed to work in my testing. Hoping to get
other's feedback.
