Remove submodules and move useful files of lib* libraries into the repo

[AntoineRondelet]

Let's remove all the code we do not use in the lib{snark|ff|fqfft} libraries, remove the submodules and move the code into the repo.

[AntoineRondelet]

I'm still not convinced about the best way to manage deps...
We need to think more about how we plan to manage dependencies in the future. I'm not aware of any decent and widely adopted package manager in cpp... There are several options to bypass that. One being to import the files of interest in the project directly, or to use git submodules for instance. I've seen both approaches used in very big and popular C++ projects, so it seems that no consensus has been reached on that.

Note: Bitcoin's way to manage deps looks interesting: https://github.com/bitcoin/bitcoin/blob/master/depends/description.md we'd need to look into that with more attention - although the project is still small enough to justify the use of submodules at this stage (cc: @dtebbs)