fix: restrict actor exit codes to non-system codes

[alanshaw]

This PR restricts actor methods from exiting with a system exit code. It changes Runtime.Abortf to only accept non-system and non-negative exit codes. If an actor attempts to exit with a system exit code it is intercepted and converted to a SysErrorIllegalActor code.

Instances where Runtime.Abortf was being called with a system error code have been changed to call a new package private function vmabortf, which allows aborting with all error codes.

The test vectors added here assert that these changes do prevent actors from exiting with system exit codes.

refs filecoin-project/test-vectors#15

[raulk]

I think this is heading in a good direction! Let's loop in @Kubuxu for an early review.

[Kubuxu]

SGWM

[whyrusleeping]

This is a consensus breaking change I believe

[Kubuxu]

@alanshaw another red label for you

[Stebalien]

I don't think this can ever work. E.g.:

1. A invokes some method on actor B.
2. Actor B fails with a system exit code. E.g., because actor A isn't allowed to call it.
3. Actor A may then "propagate" the abort and exit with the same code.

[Stebalien]

(please reopen if I'm wrong)