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

consensus/types/round_state_test.go

@@ -16,7 +16,7 @@ func BenchmarkRoundStateDeepCopy(b *testing.B) {
 	// Random validators
 	nval, ntxs := 100, 100
 	vset, _ := types.RandValidatorSet(nval, 1)
-	precommits := make([]*types.Vote, nval)
+	precommits := make([]*types.CommitSig, nval)
 	blockID := types.BlockID{
 		Hash: cmn.RandBytes(20),
 		PartsHeader: types.PartSetHeader{
@@ -25,12 +25,12 @@ func BenchmarkRoundStateDeepCopy(b *testing.B) {
 	}
 	sig := make([]byte, ed25519.SignatureSize)
 	for i := 0; i < nval; i++ {
-		precommits[i] = &types.Vote{
+		precommits[i] = (&types.Vote{
 			ValidatorAddress: types.Address(cmn.RandBytes(20)),
 			Timestamp:        tmtime.Now(),
 			BlockID:          blockID,
 			Signature:        sig,
-		}
+		}).CommitSig()
 	}
 	txs := make([]types.Tx, ntxs)
 	for i := 0; i < ntxs; i++ {