Cross testing against a naïve implementation

[BerryYoghurt]

Hi. I am an SoB intern working with @real-or-random as my mentor.

After reading #691 and going through the links provided there, I thought I could pick up where #641 left off.
In particular, I am planning to transpile the Rust library to C.

I know of two approaches to transpile Rust to C:

1. Emit LLVM using rustc then use llvm-cbe to generate C.
2. Use mrustc to generate C directly.

The LLVM approach seems not to work (neither clang nor gcc are happy with the generated C). So mrustc seems to be the way to go.

If anybody knows of a third way, I'd be happy to explore it too, just in case mrustc fails midway

[BerryYoghurt]

mrustc has some parsing issues, but I haven't given up on it yet.

But I was thinking I could also emit assembly from cargo and then decompile this assembly to C, but I am not sure how feasible this is. I'd appreciate it if somebody could suggest tools or readings that might help with this.

[BerryYoghurt]

It turns out that using mrustc to transpile ecc-scep356k1 is not feasible.
In order to use mrustc's transpiled output, we have to:

1. Either vendor the transpiled version of libstd along with the transpiled ecc-scep356k1 (and this transpiled code is not cross-platform at that). This would bloat up the code added for cross-testing, and wouldn't be a very clean or maintainable solution.
2. Or we have to re-write ecc-scep356k1 to not use anything from libcore. And I don't think the (machine-dependent and unreadable) end result would incentivize such an endeavor.

So, @elichai, thank you for promptly removing the GMP dependency, but unfortunately I've reached a dead-end and will not continue using ecc-scep356k1.

Moving forward, I am now planning on using libecc for cross-testing directly in C.