types.NewCommit (#3275)

* types.NewCommit

* use types.NewCommit everywhere

* fix log in unsafe_reset

* memoize height and round in constructor

* notes about deprecating toVote

* bring back memoizeHeightRound

consensus/types/round_state_test.go

@@ -53,11 +53,8 @@ func BenchmarkRoundStateDeepCopy(b *testing.B) {
 		Data: types.Data{
 			Txs: txs,
 		},
-		Evidence: types.EvidenceData{},
-		LastCommit: &types.Commit{
-			BlockID:    blockID,
-			Precommits: precommits,
-		},
+		Evidence:   types.EvidenceData{},
+		LastCommit: types.NewCommit(blockID, precommits),
 	}
 	parts := block.MakePartSet(4096)
 	// Random Proposal