Safe and Assertion removed

self returned by TOTP create method

* Leeway

* Doc

* Rector+ECS fix

* Minimum PHP version

* Rector rule restored

* Leeway
* Doc

* Update TOTP.php
* Update TOTPInterface.php
* Tests

* More comprehensive code example. Includes details in how to
  retrieve the generated secret, and how to create OTP object
  from the stored secret.

* Update composer.json
* PHP 7.4 added to travis-ci
* PHPDoc and Typehint fixed

Changed hash_hmac to return raw binary, removing the need for hex2bin
Changed result of unpack() to be 0 based for better code readability

* Timecode calculation method corrected
* Fixed typo (#92)
    * otherwize -> otherwise
* Minor changes
* Uses newer syntax and uses unpack instead of splitting strings
* Add type hint