Task Description: the function PackFile_unpack in src/packfile/api.c is large and bloated. We need to refactor it out into smaller chunks, and make each chunk an individual function.

Break PackFile_unpack into several small static functions with descriptive names, following logical code boundaries. Where possible, try to make the new functions reusable: A function to verify the integrity of a general packfile is preferred over a function which does partial verification, or some verification and some unpacking, etc. This issue is discussed in Parrot ticket #749. Comment with your progress in that ticket.

Steps to Complete:

1. Create a fork of parrot/parrot on Github
2. Refactor PackFile_unpack into several functions.
3. Build parrot and run all tests
4. open a pull request on Github