Split VariableInspector inner classes into multiple files

Also rename VariableEntry to Variable.
Track usage of each assignment in UnusedLocalVariable

The cop now tracks usage of each assignment
rather than usage in entire variable lifetime.

This closes #458.
Fix crashes at class definition subclassing class stored in a lvar
Reorganize UnusedLocalVariable and UselessAssignment

* Extracted useless setter call tracking part of UselessAssignment to
  UselessSetterCall.
* Renamed UnusedLocalVariable to UselessAssignment.
Merge pull request #469 from yujinakayama/reassigned-but-unused-variable

Track usage of every assignment in UnusedLocalVariable