custom serializer

This is my draft for the custom serializer.

I tried to mimic your coding style. I did not define a default sub implementing the default freeze/thaw algorithm but having a ternary ?: like the table name  accessor in new freeze and thaw method.

Defining a custom serializer is "initialization only", both freeze and thaw must be defined. There is no check which checks for a valid code ref yet.

31-enqueue-json.t is copy of 30-enqueue.t with a few changes and a new dependency to JSON::XS. 
Maybe we should have a custom test algorithm which is implemented without adding additional dependencies, for example a reverse Base64 encoding.