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/block_test.go

@@ -198,7 +198,6 @@ func TestCommit(t *testing.T) {
 	commit, err := MakeCommit(lastID, h-1, 1, voteSet, vals)
 	require.NoError(t, err)
 
-	assert.NotNil(t, commit.FirstPrecommit())
 	assert.Equal(t, h-1, commit.Height())
 	assert.Equal(t, 1, commit.Round())
 	assert.Equal(t, PrecommitType, SignedMsgType(commit.Type()))