Add ssgen vote helpers in txhelpers.
Add ssgen vote helpers in txhelpers.

Functions added:
- SSGenVoteBlockValid
- VoteBitsInBlock
- SSGenVoteBits
- SSGenVoteChoices

structs added:
- BlockValidation
- VoteChoice

Add deployment/vote/choice indexes to the VoteChoice struct.

Add docs for BlockValidation and VoteChoice.

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