Implement correctly Settable inspectors

- Array inspector implements correctly set and resize
- Map inspector implements settable
- Root (and struct) inspector implements settable
- Inspectors will now inspect basic objects because Hive sometimes
does that
Extract primitive inspectors and instantiate them only once
Fix #177: Inspect key when accessing maps
Add some javadoc to clarify
Inspect keys only for a few types in parquet hive maps