Init fuzzing harnesses for scalars and points

[integritychain]

This PR contains initial-pass fuzzing harnesses for scalars and points across each of the bign256, k256, p192, p224, p256, p384, p521, and sm2 curves. I thought I would offer up the current state and confirm there is an interest before next implementing keys and signatures.

Other than adding the fuzz workspace there are no code changes -- just two fairly straightforward harnesses plus a README.md. I review cryptography code as my day job and find deserialization issues all too frequently, even in Rust code. I have indirectly worked on the k256 code in the past as first author.

There is future potential for connecting this into the OSS-Fuzz project.