This should've been a part of
https://github.com/tendermint/tendermint/pull/3960, but I forgot about
it while reviewing.

A good programmer is someone who always looks both ways before crossing
a one-way street. - Doug Linder

* VoteSignBytes builds CanonicalVote

* CommitVotes implements VoteSetReader

- new CommitVotes struct holds both the Commit and the ValidatorSet and
  implements VoteSetReader
- ToVote takes a ValidatorSet

* fix TestCommit

* use CommitSig.BlockID

Commits may include votes for a different BlockID, could be nil,
or different altogether. This means we can't use `commit.BlockID`
for reconstructing the sign bytes, since up to -1/3 of the commits
might be for independent BlockIDs. This means CommitSig will need to
include an indicator for what BlockID it signed - if it's not the
committed one or nil, it will need to include it fully in order to be
verified. This is unfortunate but unavoidable so long as we include
votes for non-committed BlockIDs (which we do to track validator
liveness)

* fixes from review

* remove CommitVotes. CommitSig contains address

* remove commit.canonicalVote method

* toVote -> getVote, takes valIdx

* update adr-025

* commit.ToVoteSet -> CommitToVoteSet

* add test

* fix from review

* Remove deprecated PanicXXX functions from codebase

As per discussion over
[here](https://github.com/tendermint/tendermint/pull/3456#discussion_r278423492),
we need to remove these `PanicXXX` functions and eliminate our
dependence on them. In this PR, each and every `PanicXXX` function call
is replaced with a simple `panic` call.

* add a changelog entry

* types.NewCommit

* use types.NewCommit everywhere

* fix log in unsafe_reset

* memoize height and round in constructor

* notes about deprecating toVote

* bring back memoizeHeightRound

* 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

This condition is really hard to get. Initially, lockedRound and
validRound are set to -1 as we start with round 0.

Refs #2702

* 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

Signatures are now []byte, which saves on the number of bytes after
amino encoding

(squash this) address Ismail's comment

This reverts commit d02c5d1e30.

Also, fix consensus liveness issue.

This reverts commit 96a3502

* use increment and decrement operators.

* remove unnecessary else branches.

* fix package comment with leading space.

* fix receiver names.

* fix error strings.

* remove omittable code.

* remove redundant return statement.

* Revert changes (code is generated.)

* use cfg as receiver name for all config-related types.

* use lsi as the receiver name for the LastSignedInfo type.

uint64 is considered dangerous. the details will follow in a blog post.