Api updates corresponding to post-0.2 `rust-umbral` PRs

[fjarri]

Already merged rust-umbral PRs:

• Made SecretKey and SecretKeyFactory serializable via to_secret_bytes() instead of bytes(), and added a SerializableSecret base class (https://github.com/nucypher/rust-umbral/pull/53)
• Removed SecretKey.__eq__() (https://github.com/nucypher/rust-umbral/pull/53)
• Added VerifiedCapsuleFrag.from_verified_bytes() (https://github.com/nucypher/rust-umbral/pull/63)
• Changed the seed size in SecretKeyFactory from 64 to 32 bytes (https://github.com/nucypher/rust-umbral/pull/64)
• Added SecretKeyFactory.secret_key_factory_by_label() (https://github.com/nucypher/rust-umbral/pull/64)
• Added SecretKeyFactory.from_secure_randomness() and .seed_size() (https://github.com/nucypher/rust-umbral/pull/64)

[fjarri]

The discussion about hashing to scalars is happening in https://github.com/nucypher/rust-umbral/issues/35 . The rest is ok to mere since https://github.com/nucypher/rust-umbral/pull/64 is merged.