Proposed 0.8 branch

[phpfui]

Proposed new branch 0.8

Drops support for PHP 7.0 and lower (only .7% current usage)

• Fix depreciation of typeless function parameters with a default of null. This will probably be enforced in PHP 9.0, so this change future proofs the library. The previous fix for 8.4 did not solve the problem.
• Drop support for 5.3 through 7.0. The current usage statistics from Packagist.org suggest no one is using PHP 5.3, 5.4 and 5.5. Usage of 5.6 and 7.0 is 0.7%. Users of 7.0 and lower can use the 0.7 branch.
• Drop support for PHPUnit less than 7.0. They are no longer supported anyway and no longer needed after dropping PHP 7.0 and lower support.
• Removed tests for InvalidArgumentException exceptions, as this is now enforced by PHP itself.
• Added return types for all functions including void based on DocBlocks or code inspection.
• Added typed parameters with null support for all functions based on DocBlocks
• Removed useless DocBlock comments when they just provided type information, as that information is now in the PHP code.

[mickverm]

I'm a fan of these changes, but I'm not sure why this should become a 0.8 if 0.7 hasn't even been released yet.

[phpfui]

I did not realize 0.7 was not released yet. I am not a direct user of this library, just another dependency for me.

But 0.7 works!

[mickverm]

@WyriHaximus I see you last worked on this project, any chance you could have a look at this PR?