PSR-16 Compatability

[PHLAK]

Stash should implement PSR-16 to be cross-compatible with other caching libraries.

See: https://www.php-fig.org/psr/psr-16/

[PHLAK]

Example package: https://github.com/php-fig/simple-cache (https://packagist.org/packages/psr/simple-cache)