Refactor includes

[aleksejspopovs]

This pull request is related to pull request 7 in libff.

This change makes it clear and explicit when files being included come from an external library, libff.

This pull request (as well as the accompanying ones in libsnark and libff) was largely generated using a Python script.

[aleksejspopovs]

I updated this pull request, so it is now relevant again in the context of pull request 8 in libff.

[howardwu]

LGTM. Builds and passes tests - make check output:

Test project /home/vagrant/libfqfft-a89c54a27dfb2d8182ed29c41a6e36b1fca8f238/build
    Start 1: gtests
1/1 Test #1: gtests ...........................   Passed    0.04 sec

100% tests passed, 0 tests failed out of 1

Total Test time (real) =   0.10 sec
[100%] Built target check