Support PHP 8

[lstrojny]

Support for PHP 8 based on @phanan’s PR #236

Todos:

• Upgrade PHPUnit
• Update tests to work with PHPUnit's method changes, removals, and deprecations
• Upgrade PHP-CS-Fixer
  • Temporarily allow PHP-CS-Fixer to fail for PHP~8, as the tool doesn't support it yet
• Change the AbstractTestCase::expectArgumentError method to adapt to the new message format thrown in PHP 8 (actually, create a new method altogether).
• Fix the few broken tests, which might come from broken functionalities themselves. @lstrojny maybe can lend a hand here?
• Opt out for named parameters
• Figure out backwards compatibility

[phanan]

Do we or do we not import the root namespaced functions? It looks like a mix at the moment.

[lstrojny]

It’s a mess, yeah. Functional shows it's age here (I just realized when updating the copyright notice it’s 10 years anniversary). When it started, use function … was not a thing. But yes, it should be imported everywhere.

Edit: I misread the question.

To the actual question: no, we backslash-ref import them. php-cs-fixer does it consistently for us. Still want to add php-cs-fixer to CI.

[lstrojny]

Most of these attributes can be removed, as their values are the same as the default.

Good point, done