Fix foldLeft and foldRight return types

[axlon]

The following code is valid, but not allowed by template types currently. This PR fixes that.

$some = new Some(5);
$result = $some->foldLeft(1, function($a, $b) { return (string) ($a + $b) }); // string("6")

[ojhaujjwal]

@GrahamCampbell Can we get this merged please if all okay?

plus my PR too #78 around the same issue.

[GrahamCampbell]

Thanks for spotting this, and you're right that the current annotation is too strict. As written though, @return R overshoots in the other direction: None::foldLeft() returns the initial value at runtime, so the true return type is S|R, and PHPStan flags None itself under this change. I've put up #82 which types the base methods as S|R with exact overrides on Some and None, so closing this in favour of that.

[GrahamCampbell]

The current @return S on the folds is indeed too strict, but changing it to @return R isn't right either: Some returns the callback result, while None just hands back the initial value, so the honest return type is the union of the two. Annotating R alone would have PHPStan bless code that blows up on the None path, and it fails the library's own analysis run. I've fixed this in #82 by returning S|R on the base class with exact overrides on Some and None. Thanks for spotting the problem and the nudge in the right direction.