Simplify compilation on different platforms

[Solpatium]

Please note that I am not a C++ expert, so I'll appreciate all feedback.
I included all changes necessary to get it working on my machine, but in the meantime some problems were fixed by other PRs. Starknet.py uses this code to build native code (https://github.com/software-mansion/starknet.py).

Changes:

• Added gtest as recommended in https://google.github.io/googletest/quickstart-cmake.html and added gmock to target_link_libraries for tests.
• Made src/starkware/crypto/ffi/js/CMakeLists.txt copy either .so or .dylib files.
• Added portable endian header to allow compillation of platforms that don't support endian.h based on https://github.com/sahlberg/libsmb2/blob/master/include/portable-endian.h. support for building on Apple #4 includes headers for Apple devices.
• Replaced bn with bn.js in js package to fix js tests. fix: install bn.js to fix js test error #7 fixes it as well.
• Added uint128 substitute for 32 bit platforms.