Adds Psr6CacheItemPool (allowing use of e.g. symfony/cache - see #4 )
Changes internal cache delimiter to be PSR6 friendly ( : => _ - this shouldn't affect anyone)
Update github actions + unit tests

• Drop support for PHP < 8.0
• Adds type hints and return types etc
• Updates a few libraries

See also #6

• build on github actions for PHP '7.2', '7.3', '7.4', '8.0', '8.1' & '8.2'
• psalm/phpdoc fixes