Switch to Sha256 and port over go-cpace tests

[BlackHoleFox]

I finally had the time to take a look back at this repo and I was happy to see that the Go implementation switched over the SHA256 so some of cpace's XXX markers could be resolved. In the process of switching to SHA256, I also ported over all of the "basic tests" that were applicable from Filippo's implementation.

I also added the transcript test. I mocked the RNG by dumping some bytes in hex from the Go test and then using that as a "entropy pool" for a faked RNG that could act in place of OsRng. So now at least we know, and backed by a test, that it is interoperable with another implementation.

Closes #1, #2, #3