Execution proofs

At the moment result of a `call` instruction isn't checked by the interpreter. And moreover a node doesn't sign it.

To make system more sustainable it needs to introduce some form of crypto proofs.

Start by reading the interpreter's handling of the `call` instruction and determine how its result is currently treated. The issue does not name files, tests, or a proof scheme, so the required cryptographic design and completion criteria need clarification before implementation can begin.

Written by the indexing model from the issue text.