Turn stake table impl generic over key-value types

[alxiong]

See #66 (comment)

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update: after some local attempts and discussion with @mrain, we realize that generic over V would cause unnecessary code complexity.
(particularly, u256_to_field() and sample() that internally sample a U512 first, and mod into U256 for negligible non-uniformity)
given this stake table is only used in hotshot, we gonna stick with concrete U256 type for now, and only generic over key type K.