Apply PHP 8.2's `SensitiveParameter` attribute to all string parameters

Add missing backslash in front of `pack()` call

Improve performance of the hex encoder

Update .gitattributes

This is a strict decoding method that doesn't tolerate '=' padding.

Resolves #22.

Single bytes can more efficiently be accessed by directly indexing the string.

This also implicitly fixes a bug without any visible effect: `encodeUpper`
retrieved two characters from the bytestring, but only ever used the first.

PHP 7.0 doesn't support nullable types, so we can't type-hint this as ?int. If we declare it as int, users can't explicitly pass NULL.

Clean up unnecessary annotations

GitHub actions improvements and fixes

- Run actions on pull request
- Run actions with PHP 8.1 as well
- Fixes for Psalm failures
- Use Psalm provided by tools
- Run PHPUnit on correct PHP and PHPUnit versions

Add .gitattributes file to ignore paths when exporting

Switch from Travis CI to Github Actions

Show users the proper way of importing classes

We have to remove 7.0 since Travis uses Composer 2 and our dependencies will not install on PHP 7.0

Fix exception message