Minimize patches to third party code

[dgarbuzov]

Some of the patches (for GraphBasics, compcert) include a large amount of additional code. Keeping it around in a patch file makes it difficult to edit and track versions. We should move as much of it as possible into separate Coq files in the repository.

[jeehoonkang]

I think maybe git-module feature helps in this case. More specifically,

• Clone the Coq-Equations repository in vellvm organisation.
• Modify the repo.
• Make a "soft link" to the cloned Coq-Equations repo by git-module.

More reference at http://git-scm.com/book/en/Git-Tools-Submodules