Fix bug in epoch second parsing and improve tests (#834)

* Implement time format/parse test suite generator

* Run generated test suite against `aws-smithy-types`

* Fix minor difference with test suite HTTP date formatting

* Fix bugs discovered in epoch seconds parsing

* Reduce allocations in epoch seconds formatting

* Update changelogs

* Improve test data format

* Remove unsafe from epoch-seconds parsing and increase test coverage

* Add fuzz test suite for date-time parsing to `aws-smithy-types`