templates and iterators in stacktraces

## proposal
if problem is hard or expensive, 1 practical approach would be a MRU cyclic buffer of template/iterators most recently calls so that at least we know what was called recently

unless there's an easy way to find all the exits of a template/iterator

## links
* D20210807T151245
* nkComesFrom
* https://github.com/nim-lang/compilerdev/issues/11
