Api fixes

[fjarri]

Based on top of #218

Type of PR:

• Refactor

Required reviews:

How many reviews does the PR author need?

• 2

What this does:

• Removes Ethereum addresses from the API completely since they play no role in the cryptographic operations, and are not even serialized (Validator structures are transient; the Rust Validator impls Serialize, but the serialization is not exposed in the bindings). If the user wants to associate addresses with validators, they can do it in their own code.
• Removes me from Dkg. None of its methods beside create_share ones require me, and the share creation ones already get the validator keypair, so we can locate the share index by the public key. (seems like it makes ferveo-python/examples/exception.py meaningless?)
• Removes the public_key field from AggregatedTranscript (use public_key() method instead). Fixes api::AggregatedTranscript serialization issue in nucypher/nucypher #209

[fjarri]

I can include a fix for #209 here too.