Introduce CommitSig alias for Vote in Commit (#3245)

* 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

types/vote_set.go

@@ -541,11 +541,13 @@ func (voteSet *VoteSet) MakeCommit() *Commit {
 	}
 
 	// For every validator, get the precommit
-	votesCopy := make([]*Vote, len(voteSet.votes))
-	copy(votesCopy, voteSet.votes)
+	commitSigs := make([]*CommitSig, len(voteSet.votes))
+	for i, v := range voteSet.votes {
+		commitSigs[i] = v.CommitSig()
+	}
 	return &Commit{
 		BlockID:    *voteSet.maj23,
-		Precommits: votesCopy,
+		Precommits: commitSigs,
 	}
 }