com.redhat.asdl.test.python2.FreeVariables: Reorganize

Preparation for statement handling and write tracking.
com.redhat.asdl.test.python2.FreeVariables: Track read and write access

This prepares for handling statements.
com.redhat.asdl.test.python2.FreeVariables: Separate writing visitor
com.redhat.asdl.test.python2.FreeVariables: Handle statements
com.redhat.asdl.test.python2.FreeVariables.Access.LOCAL: Renamed

Previous name was WRITE.