updated to 0.17

ChangeLog:
0.17
- More void context correctness fixes WRT tied values
- Overzealous seen value mappings made by Callback were removed

0.16
- Fix passing of void context when visiting hashes/arrays (perf
  optimization to avoid unnecessary cloning)
- Added 'visit_seen' and a 'seen' callback for circular structures
- Class callbacks are now fired from least derived to most derived, not in
  hash key order

(rhaen)