impl CanonicalSerde for Powers, and PartialEq for params and keys

[zhenfeizhang]

Description

• adding serdes for Powers
• allow the callers to derive PartialEq for UniversalParams and VerifierKey

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[weikengchen]

just one quick question related to the PR:

so the PartialEq is added for serialization?

[alxiong]

No, PartialEq is for accompanying auto-derived Hash trait, motivated by cases where consistent hashing of the verifying key is needed.

For example, the predicate identifier in zexe: pid = H(vk)

[Pratyush]

LGTM modulo these small changes

[zhenfeizhang]

Thanks, and fixed.

[Pratyush]

btw, for the hashing, you should note that the Hash trait in std is not for cryptographic hashing.