Port to PHP 7.1

[phpfui]

Lots of changes. I put most in the final commit so you can see everything in one commit. This is a major improvement to the package and will make maintenance easier going forward.

I tried to add scalar parameter types and return types where ever possible. I did not type the object returns, as I am not that familiar with how the code should work and you may want to use a more abstract or concrete class depending on the situation. Also this can cause problems with type mismatches at run time, so you have to be careful.

I also did not change any logic where ever possible. This should be just specifying parameter and return types.

I did not resolve all PHPStan errors, as you should take a look, but I got most of them. I think the remaining issues are more judgement calls for you.

Things that should be considered going forward:

• Add all object return types.
• Return $this on all methods that currently return void. This allows for chaining and is fairly common in modern libraries.
• Reduce multiple return types such as ?int and ?string unless null is always an error condition. Not sure how the methods are all used.
• Reduce the number of nullable parameters. I did find a lot of mocks that were returning null or a mock object when they should probably not. So some of these issues are only test related.
• Port to 7.4 and add property types. I think some property types are already in or can be added to the doc blocks, especially the tests, but that is a minor issue.

Let me know what else needs to be done / changed.

[phpfui]

Unit test failures are all due to dropping older versions of PHP. OK to merge.

[phpfui]

Should be good now, I thought I removed old PHP versions, but apparently on another branch I trashed.

[zbateson]

This is amazing work, thank you very much!