v2.2.0

* Transaction Malleability:
If you allow for both values 0/1 and 27/28, you allow two different
signatures both resulting in a same valid recovery. (r,s,0/1) and
(r,s,27/28) would both be valid, recover the same public key and sign
the same data. Furthermore, given (r,s,0/1), (r,s,27/28) can be
constructed by anyone.

* Transaction Malleability:
EIP-2 still allows signature malleabality for ecrecover(), remove this
possibility and force the signature to be unique.

* Added a reference to appendix F to the yellow paper and improved
comment.

* better test description for testing the version 0, which returns
a zero address

* Check that the conversion from 0/1 to 27/28 only happens if its 0/1

* improved formatting

* Refactor ECDSA code a bit.

* Refactor ECDSA tests a bit.

* Add changelog entry.

* Add high-s check test.

* Add extcodesize check to SafeERC20.

* Clarify some comments.

* Replace inline assembly with Address.sol.

* Add TimedCrowdsale::_extendTime

* Add tests for TimedCrowdsale extending method

* Reverse event arguments order

* Rename method argument

* Refactor TimedCrowdsale test

* Simplify TimedCrowdsaleImpl

* Fix extendTime method behaviour to deny TimedCrowdsale re-opening after it was ended

* Append chengelog

* Update CHANGELOG.md

Co-Authored-By: k06a <k06aaa@gmail.com>

* Update contracts/crowdsale/validation/TimedCrowdsale.sol

Co-Authored-By: k06a <k06aaa@gmail.com>

* Improve tests

* Add guard to ERC20Migrator migrate function

* Add tests for premature migration in ERC20Migrator

These tests apply to the new guard condition, but they don't
fail without it, since the functions revert anyway.
They are added for completeness and to ensure full code coverage.

* Use context block around premature migration tests

We should use context blocks for situational details
and describe for features or functions.

* rename previous audit to date it was performed

* add latest audit

* add note about latest audit in README

Using "a" instead of "an" makes this consistent with the comment on `allowance`.

* Add no-return-data ERC20 support to SafeERC20.

* Add changelog entry.

* Replace abi.encodeWithSignature for encodeWithSelector.

* Remove SafeERC20 test code duplication.

* Replace assembly for abi.decode.

* Fix linter errors.

* Add IntelliJ IDE config to .gitignore

* Fix variable name in ERC20 function comments

* Fix typos in Arrays function comment

* Fix typos in ownership test names

* Fix typo in Pausable test name

* Fix grammar in Ownable function comment

* Fix grammar in Crowdsale contract comment

* Fix typo in Counters contract comment

* Fix typo in ERC721Enumerable comment

* Fix typo in ERC721PausedToken test name

* Fix typo in Crowdsale function comment

* Fix typo in IncreasingPriceCrowdsale function comment

* Fix grammar in IncreasingPriceCrowdsale test name

* Fix typo in AllowanceCrowdsale test name

* Fix typo in RefundEscrow function comment

* Fix typo in ERC20Migrator contract comment

* Fix typos in SignatureBouncer comments

* Fix typo in SignedSafeMath test name

* Fix typo in TokenVesting contract comment

* Move Ownable comment from @notice section to @dev

The Ownable contract has a comment explaining that renouncing
ownership will prevent execution of functions with the onlyOwner
modifier.

This commit moves that comment to the @dev section and replaces it
with a description suitable for a generic user.

* Clarify purpose of ERC20 transfer function

* Clarify registration of ERC721Enumerable interface

* Clarify purpose of AllowanceCrowdsale test

* Increase specificity of inheritance comments

FinalizableCrowdsale and RefundableCrowsale both have comments
indicating that they are extensions of the Crowdsale contract.

This commit refines those comments to the most immediate ancestor
( TimedCrowdsale and RefundableCrowdsale respectively )

* Remove unused parameter in PaymentSplitter test

* Rename parameter in SignatureBouncer functions

The SignatureBouncer contract has modifiers to validate the
message sender is authorised to perform an action. They pass
msg.sender to internal functions as the variable `account`, but
the function comments refer to the variable as `sender`

This commit changes the variable name to `sender`

* Clarify comments in SignatureBouncer functions

The SignatureBouncer has comments that use the description
`sender` to refer to the variable `account`.

This commit updates the comments for consistency.

Maintainer Note: this reverts changes in the previous commit,
which renamed the variable `account` instead.

* Add the solidity linter command to the PR template

The PR template states that a contributor should run the
Solidity/JS linters before submission. However, it only states the
command for the JS linter.

This commit adds the Solidity linter command explicitly.

* Use past tense in the list of prerequisites

Fix SafeERC20.safeApprove bug

(cherry picked from commit 3111291b)

Fix SafeERC20.safeApprove bug

(cherry picked from commit 3111291b)

Added PR links for 2.2.0 changelog entries.

Fix SafeERC20.safeApprove bug

Use canonical EIP reference format

Update to preferred citation formation for ERC-721

Add a link to the minime token

Fix typo: snapshoted