Ensure everything compiles individually & sort includes

[aleksejspopovs]

A lot of the files in our codebase don't properly include all of their dependencies. A common example of this is a situation like this: a.hpp uses std::vector in a definition, so it would need to have a line saying #include <vector>, but it doesn't. a.hpp does not include vector neither directly nor indirectly, but whenever a.hpp is included from another file, that file just happens to include vector beforehand.

The result is that, while running make succeeds, the code is quite brittle. The fact that vector is always included before a.hpp (in current uses of the library) might be quite accidental, and users might in theory want to use a.hpp in another file that doesn't need vector itself. Besides, it's just not particularly good style to not include our dependencies explicitly.

Luckily, this specific problem is easy to detect: if you were to try to compile a.hpp on its own, compilation would not succeed. I wrote a script (try_compiling.py in libsnark-tooling) that tries to individually compile every single file (*.hpp or *.cpp) in libff and reports whether it succeeds.

I then went through the report and fixed every single compilation error --- almost all of them were because of missing includes.

This does not resolve all possible include-related problems: in the situation above, perhaps a.hpp actually has a line saying #include <libff/b.hpp>, and b.hpp includes vector --- then a.hpp compiles just fine, but is still not explicitly listing all of its dependencies (perhaps b.hpp's dependencies will change in the future, and it will no longer require vector --- then we'll get a broken a.hpp despite no real changes inside it). But I think this is a much smaller problem, and it's also much harder to even detect automatically, so for now I'm submitting just these changes.

While I was at it, I also sorted the #include directives in every file --- they are now grouped into three groups (standard library, external libraries, and libff) and sorted lexicographically within each group. This was done using fix-includes.py in libsnark-tooling.

Another minor change I made is that libff/common/default_types/ec_pp.hpp will now refuse to compile (with a helpful error message) if none of the CURVE_* symbols are defined, because that file is always included with the expectation that it will define default_ec_pp.

[tromer]

Looks great, and indeed needed. Let's merge.