Changed File::Slurp to File::Slurp::Tiny

Use of `File::Slurp` is discouraged (e.g. see [this](https://rt.cpan.org/Public/Bug/Display.html?id=83126)). This PR changes this to [File::Slurp::Tiny](https://metacpan.org/pod/File::Slurp::Tiny).

This PR is part of my [advent quest](https://questhub.io/realm/perl/quest/547b4202070ccf750d00010e).

Cheers,
Sergey.