Allow effectful upgrading

[lortabac]

We should provide a way to upgrade effectfully. The use cases that come to my mind are reading the default values from a database, keeping some state or logging/monitoring.

I am not sure what the best way is to add this functionality. The simplest solution would be to add a new UpgradeM module and let the users choose between pure and monadic upgrading by importing the module they need. The downside is that it would become possible to define instances for both upgrading modes, which may be a bad idea.

Unfortunately I was not able to find a way to unify the interface without making pure upgrading more cumbersome.

[YoEight]

I strongly believe you should provide a simple function that can drive the upgrading process 'effectfuly'. Use a record representing all possible stages occurring during an upgrade so a user can plug effectful actions during those stages.