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.go

@@ -59,6 +59,16 @@ type Vote struct {
 	Signature        []byte        `json:"signature"`
 }
 
+// CommitSig converts the Vote to a CommitSig.
+// If the Vote is nil, the CommitSig will be nil.
+func (vote *Vote) CommitSig() *CommitSig {
+	if vote == nil {
+		return nil
+	}
+	cs := CommitSig(*vote)
+	return &cs
+}
+
 func (vote *Vote) SignBytes(chainID string) []byte {
 	bz, err := cdc.MarshalBinaryLengthPrefixed(CanonicalizeVote(chainID, vote))
 	if err != nil {