Ethsnarks for Windows, and AppVeyor continuous integration about ethsnarks

[HarryR]

Building with msys/mingw might be easier:

• https://github.com/IlyaFinkelshteyn/tpch-dbgen/blob/158092d05dc7fe827ae271107cade1e8f8a5f7f6/appveyor.yml
• https://stackoverflow.com/questions/48500554/trouble-with-my-appveyor-windows-build-for-a-simple-c-project-on-github
• https://cmake.org/cmake/help/v3.1/generator/MinGW%20Makefiles.html
• https://stackoverflow.com/questions/46022210/how-to-configure-cmake-for-msys2
• https://www.scivision.co/windows-gcc-gfortran-cmake-make-install/

However, need to install the GMP and OpenSSL components via msys, or alternatively just build them separately and then link against them statically.

Come to think about it - is it better to do static builds of OpenSSL and GMP?

A very limited subset of OpenSSL / libcrypto functionality is used, and for libmiximus.(dll|dylib|so) no OpenSSL functionality is used at all - only GMP (see nm -D libmiximus.so)

Paths for Appveyor: https://www.appveyor.com/docs/build-environment/

But.. it would be nice to have a single file for build_on_windows.bat or build_on_windows.ps

[HarryR]

I've got it almost building on appveyor using MSYS, but it runs into an error:

[ 22%] Built target ethsnarks_gadgets
src/CMakeFiles/hashpreimage.dir/build.make:86: *** target pattern contains no '%'.  Stop.
make[1]: *** [CMakeFiles/Makefile2:1086: src/CMakeFiles/hashpreimage.dir/all] Error 2
make: *** [Makefile:95: all] Error 2

See: https://ci.appveyor.com/project/harryr/ethsnarks/builds/21534551 for full log

It won't build with MSVC because there's no GMP library, but mingw/msys have one.

Most C++ tests are run via cmake now, and the appveyor.yml file should run the tests after a successful build.

[HarryR]

The previous build error has been fixed, this was due to GMP::gmp-NOTFOUND being used as a target name in a Makefile (even though it's found on OSX and Linux, so have directly referenced the ${GMP_LIBRARY} variable instead of the meta-library).

The project now builds successfully on Windows, see: https://ci.appveyor.com/project/harryr/ethsnarks

However... most if not all of the tests fail for mysterious reasons, and one hangs indefinitely. I think this is related to libff and some mysterious weirdness, e.g.:

Program: C:\projects\ethsnarks\build\src\test\test_subadd.exe
File: C:/projects/ethsnarks/depends/libsnark/libsnark/gadgetlib1/gadgets/basic_gadgets.tcc, Line 50
Expression: this->pb.lc_val(packed).as_bigint().num_bits() <= bits.size()

Additionally all zkSNARK proofs fail to verify.

It seems something to do with the finite field library using the wrong field prime.

For now, the tests have been disabled. But a full log of the failing tests is available at: https://ci.appveyor.com/project/harryr/ethsnarks/builds/21950411