Adding types to returns and arguments

[MarcHagen]

So this should be a major version, I guess, because it could break all installations.

However, 7.0 <> 7.4 is deprecated and it is a good thing to maybe move to php: ^8.0 so we can add a little bit of strictness to the code.

What are your thoughts on this?

[ericsizemore]

@WyriHaximus this issue could be closed with #88 being merged.