Move extra methods from actor states

[LesnyRumcajs]

Issue summary

Actor state files must remain as close to their source as possible. They should not be extended by additional methods directly like in #290. All such occurrences should be moved to, e.g., Forest shim or extension traits.

This is should lower the upgrade burden (less manual copy-pasting from our own definitions - only from the builtin-actors) and manage our expectations for the given methods (builtin-actors code is the holy grail and we should assume it's correct, otherwise it's a massive issue for Filecoin networks).

Other information and links

[LesnyRumcajs]

@sudo-shashank Could you please identify which methods are "our own" and should be migrated elsewhere?

[sudo-shashank]

@sudo-shashank Could you please identify which methods are "our own" and should be migrated elsewhere?

okay sure