Refactor ThresholdDecryptionResponse

[cygnusv]

ThresholdDecryptionResponse() can take a DecryptionShareSimple object itself, not its bytes. Correspondingly, its property will also be typed. Can be a union type to support different share types.

[fjarri]

Some problems:

• The proper way (making the Rust nucypher_core::ThresholdDecryptionResponse struct have a typed decryption_share field) changes the ABI and thus cannot be done at the moment.
• Fixing only the Python signature is also impossible since the DecryptionShareSimple serialization is internal to ferveo (and different from what nucypher-core uses) and cannot be accessed. We can replicate it, but it just adds more possibilities for future errors.
• It's possible to call Python bytes() from Rust to serialize the share in ThresholdDecryptionResponse::new(), and for the property attempt to deserialize in the same way into a simple or into a precomputed share, but that's just too ugly.

So this will have to wait.