ART: Split Checker into smaller files

Checker code has become too messy and incomprehensible. This patch
splits it into more manageable and better structured units.
Functionality remains unchanged.

Resubmission of change I870c69827d2be2d09196a51382a3f47f31cd2ba3 due
to omission of file 'tools/checker/file_format/common.py'.

Change-Id: I277a4aa65a2e3b54f0e89901fdb9f289f55a325f