FIP-0048: Add an f4 address class FIP

[Stebalien]

Support new actor addressing schemes and interactions with addresses that do not yet exist on-chain by introducing a new address type where sub-namespaces within this new address type can be managed by user-deployed actors called "address managers".

This FIP adds a new extensible address class (f4) where an address management actor will be able to create new actors with f4 addresses under a specific f4 address prefix, in addition to the f2 address assigned today. This will allow Filecoin to support the native addressing schemes of foreign runtimes like the EVM and support interactions (both actually and counterfactually) with addresses that do not yet exist on-chain..

---

Discussion: #473

Since that discussion:

• It has been refactored to better fit the FIP layout.
• Bind4 and address forwarding have been removed as discussed.
• A checksum has been added to the textual format.
• The FEVM specific parts have been moved to the "discussion" and the specifics have been punted to a future FIP to keep this one digestible.
• Most mentions of "counterfactual interactions" have been replaced with the more accurate "interactions with addresses that do not yet exist on-chain" (with some clarifications that we're referring to both actual and counterfactual interactions).

[anorth]

Great, draft, thank you.

[debjeet214]

tell how to convert the F1 address to F4 address