[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

types/test_util.go

@@ -16,7 +16,7 @@ func MakeCommit(blockID BlockID, height int64, round int,
 			ValidatorIndex:   i,
 			Height:           height,
 			Round:            round,
-			Type:             VoteTypePrecommit,
+			Type:             PrecommitType,
 			BlockID:          blockID,
 			Timestamp:        tmtime.Now(),
 		}