PSR-16 support: serialization using plugin

[weierophinney]

Per a comment from @marc-mabe :

there is a serializer plugin available that can be plugged in into any storage adapters. Maybe it’s better to throw an error if the provided storage adapter doesn’t support all required data types and let the user plugin the serializer plugin into it to make it work with the psr wrapper. I think it was done that way for psr6, too

[thomasvargiu]

@marc-mabe Current implementation of PSR-16 is the same of PSR-6. Are you suggesting to remove the serialization from both decorators?

[weierophinney]

Fixed witH #161.