Dump FIRM graphs into subdirectory and ignore these files in Git

Fixes #169
Fix FIRM construction for assignment
Merge pull request #174 from jspam/061-dump-dumps

Dump FIRM graphs into subdirectory and ignore these files in Git
Merge pull request #177 from TehMillhouse/assignment_fix

Fix FIRM construction for assignment
Fix `int i = (i=3);` not working correctly
Merge pull request #179 from TehMillhouse/localvarfix

Fix `int i = (i=3);` not working correctly
Fix FIRM generation for arrays