Technical Design: become_actor syscall

[Stebalien]

Discussion in https://github.com/filecoin-project/FIPs/discussions/396.

[Stebalien]

cc @vyzo @raulk @mriise @anorth

[maciejwitowski]

@raulk do we need the implementation of this to land in Iron? If not, which milestone would it be needed in?

[raulk]

Nope, this has actually been descoped from M2.1 entirely based on CPH discussions. Rationale: this provided a pathway for an existing secp256k1 account to be upgraded into an Ethereum-aware account. Based on our current design blueprints, I believe we are segregating accounts so that the Ethereum-aware account will be automatically initialized on a send to an f420... address. I assume that that kind of account won't be able to process native Filecoin transactions (hence why I'm using the term segregate), as that would allow reuse of key material for the same signature scheme, but different payload format (which weakens security, something that @Stebalien raised), but worth confirming.