[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

privval/socket_test.go

@@ -79,7 +79,7 @@ func TestSocketPVVote(t *testing.T) {
 		sc, rs  = testSetupSocketPair(t, chainID, types.NewMockPV())
 
 		ts    = time.Now()
-		vType = types.VoteTypePrecommit
+		vType = types.PrecommitType
 		want  = &types.Vote{Timestamp: ts, Type: vType}
 		have  = &types.Vote{Timestamp: ts, Type: vType}
 	)
@@ -237,7 +237,7 @@ func TestRemoteSignVoteErrors(t *testing.T) {
 		sc, rs  = testSetupSocketPair(t, chainID, types.NewErroringMockPV())
 
 		ts    = time.Now()
-		vType = types.VoteTypePrecommit
+		vType = types.PrecommitType
 		vote  = &types.Vote{Timestamp: ts, Type: vType}
 	)
 	defer sc.Stop()