Switch to Git submodules for third party dependencies

[wltan]

We currently depend on alexandru-dinu/igcc (and possibly source-academy/js-slang in future) which are GitHub repositories.

In our dockerfiles we clone the repositories when the images are being built, and use that version until the next build. However, the third party repositories might push changes that break our image, so we should migrate ASAP to a submodule implementation so that the dependency remains constant.

https://git-scm.com/book/en/v2/Git-Tools-Submodules
https://www.atlassian.com/git/tutorials/git-submodule