High level history navigation APIs for Registers

[grumbach]

Quoting @happybeing:

For many real world use cases the application needs to provide a way to navigate the history within the Register

• Implement high level history navigation APIs for Registers

[happybeing]

👍 though I'm not sure if this can be implemented for all application use cases. So I would see it as a simpler alternative to using the MerkelReg directly rather than a way to hide it completely.

Otherwise some things may be impossible or much more difficult to implement.