fixed #8: nvToFolder now supports maximum nesting depths

- maxdepth is used for lists, dicts &c.
- maxodepth is used for objects &c.
- maxmoduledepth is used for modules, which tend to be eldritch horrors
  of nesting
- defaults are 30, 3 and 2, which seem sensible enough
- anything exceeding these depths gets repr()'d

I'm not entirely happy with the defaults, but at least it doesn't
infinitely loop and/or crash due to stack overflows.