Clarify Assignment 3 traversal TODOs

[JoelYYoung]

Summary

• add the missing collectICFGPath declaration and implementation stub
• separate path collection from context-sensitive reachability
• order the Assignment 3 implementation TODOs as readSrcSnkFromFile, collectICFGPath, reachability, solveWorklist, and aliasCheck
• align the header comments with the responsibilities described in the assignment slides

Testing

• git diff --check
• not run: CMake is unavailable in the local environment