Create and build a proper crate for the SDK

[bigspider]

It should work seamlessly for both the native and riscv targets, and be easy to compile.

The C SDK could possibly be removed completely, as linking with cxlib/speculos binaries turns out to be a huge headache; for native compilation, the implementation of most of the bolos calls could be emulated with native Rust libraries, instead.

[yogh333]

Hi, I fully agree, and have reached the same conclusion while digging in the repo architecture 🤯
Could you please assign me this issue ? 🙏