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/replay_test.go

@@ -539,7 +539,7 @@ func makeBlockchainFromWAL(wal WAL) ([]*types.Block, []*types.Commit, error) {
 			if p.Type == types.PrecommitType {
 				thisBlockCommit = &types.Commit{
 					BlockID:    p.BlockID,
-					Precommits: []*types.Vote{p},
+					Precommits: []*types.CommitSig{p.CommitSig()},
 				}
 			}
 		}