FVM M2.2: Replace the Constructor method with a wasm-level "constructor" entrypoint

[Stebalien]

For some background, Filecoin "sends a message" to an actor (i.e., "calls" an FVM actor) by calling a single wasm-level invoke function. This invoke function internally dispatches on the method number.

When a user constructs a new actor:

1. The user's actor/account calls the Exec method on the init actor.
2. The init actor allocates a new actor ID/address, and records it in its address map.
3. The init actor creates a "stub" actor with empty state.
4. The init actor calls new actor's Constructor method (method 1) using the normal calling convention. Internally, this will call the invoke function on the actor's wasm module.

However, this has two significant drawbacks:

1. The new actor's constructor sees the init actor as the caller, not the user's actor.
2. The constructor has to carefully check that the caller is the init actor. Technically, the constructor can be called by anyone at runtime (although every sane actor will just reject the message).

I'd like to propose an alternative: introduce a new wasm-level function called constructor. In this new world:

1. The user's actor/account calls the Exec method on the init actor.
2. The init actor allocates a new actor ID/address, and records it in its address map.
3. The init actor asks the FVM (via a privileged syscall) to construct a new actor at the newly allocated address.
4. The FVM calls the constructor function on the actor's WASM module, exporting the user's actor from step 1 as the "caller".

Alternatives

An alternative is to:

1. Make "method 1" privileged and prevent anyone from sending a method 1 message except the init actor.
2. Allow the init actor to "send as", or somehow expose the ultimate "creator's" address to the newly created actor.

[anorth]

Ack, no objections from me to this basic idea.