* types: memoize height/round in commit instead of first vote
* types: commit.ValidateBasic in VerifyCommit
* types: new CommitSig alias for Vote
In preparation for reducing the redundancy in Commits, we introduce the
CommitSig as an alias for Vote. This is non-breaking on the protocol,
and minor breaking on the Go API, as Commit now contains a list of
CommitSig instead of Vote.
* remove dependence on ToVote
* update some comments
* fix tests
* fix tests
* fixes from review