[R4R] Fixed sized and reordered fields for Vote/Proposal/Heartbeat SignBytes (#2598)

* WIP: switching to fixed offsets for SignBytes

* add version field to sign bytes and update order

* more comments on test-cases and add a tc with a chainID

* remove amino:"write_empty" tag

- it doesn't affect if default fixed size fields ((u)int64) are
written or not
- add comment about int->int64 casting

* update CHANGELOG_PENDING

* update documentation

* add back link to issue #1622 in documentation

* remove JSON tags and add (failing test-case)

* fix failing test

* update test-vectors due to added `Type` field

* change Type field from string to byte and add new type alias

- SignedMsgType replaces VoteTypePrevote, VoteTypePrecommit and adds new
ProposalType to separate votes from proposal when signed

- update test-vectors

* fix remains from rebasing

* use SignMessageType instead of byte everywhere

* fixes from review

state/execution_test.go

@@ -64,7 +64,7 @@ func TestBeginBlockValidators(t *testing.T) {
 	prevBlockID := types.BlockID{prevHash, prevParts}
 
 	now := tmtime.Now()
-	vote0 := &types.Vote{ValidatorIndex: 0, Timestamp: now, Type: types.VoteTypePrecommit}
+	vote0 := &types.Vote{ValidatorIndex: 0, Timestamp: now, Type: types.PrecommitType}
 	vote1 := &types.Vote{ValidatorIndex: 1, Timestamp: now}
 
 	testCases := []struct {
@@ -135,7 +135,7 @@ func TestBeginBlockByzantineValidators(t *testing.T) {
 			types.TM2PB.Evidence(ev2, valSet, now)}},
 	}
 
-	vote0 := &types.Vote{ValidatorIndex: 0, Timestamp: now, Type: types.VoteTypePrecommit}
+	vote0 := &types.Vote{ValidatorIndex: 0, Timestamp: now, Type: types.PrecommitType}
 	vote1 := &types.Vote{ValidatorIndex: 1, Timestamp: now}
 	votes := []*types.Vote{vote0, vote1}
 	lastCommit := &types.Commit{BlockID: prevBlockID, Precommits: votes}