Binary compatibility with PyUmbral

[fjarri]

At the moment rust-umbral is not binary compatible with PyUmbral:

• Different hashing algorithms, and possibly a difference in their implementation in Rust
• Differences in how metadata is stored (CapsuleFrag metadata as a scalar #21)
• Some differences in the serialization format (e.g. a serialized kfrag has flags of whether delegation and receiving key were signed in two separate bytes instead of one)
• Differences in API (e.g. a capsule does not include cfrags, they come separately)
• NON_INTERACTIVE and X_COORDINATE constants are different (in PyUmbral they are taken from constant_sorrow)
• kfrag ID is a CurveScalar and not just 32 random bytes
• Maybe something else

Except for the difference in hash function implementation (if any), the rest is theoretically fixable. The question is, do we want to do it? Seems like it would be better to handle this difference at the protocol level, by implementing versioning.

[fjarri]

From the discussion: KeyFrag ID doesn't need to be a scalar. We can keep it just 32 random bytes, as it is in PyUmbral.

[fjarri]

From the discussion:

• we don't change rust-umbral, except small fixes like kfrag IDs and Possibly reduce the amount of hash functions used in the code #2
• we generate test vectors in rust-umbral
• we modify PyUmbral to match rust-umbral and check that they produce the same results.

[fjarri]

From the discussion: define NON_INTERACTIVE and X_COORDINATE as simpler values instead of bigger constants. Byte-sized, for example, for ease of implementation. But we do need to keep them.