add unified interface for accessing file system

**please review**
If we weren't running tests using --debug we could use this info to check whether tests left temp files on the filesystem using something like this:

```
! find . \( -iname ".kompile*" -o -iname ".krun*" -o -iname ".kast*" -o -iname ".ktest*" \) | egrep ".*"
```