Disallow returning `Vec` (and any `raw_ptr`) from a contract to another contract

Returning a `Vec` from one contract to another should be disallowed because the pointer from the called contract is not valid in the caller contract. The same check that we have that disallows returning `raw_ptr` from scripts should also be used in this case.

Start by locating the existing check that disallows returning raw_ptr from scripts, then trace the validation applied to contract-to-contract return values. Extend that check to cover Vec and raw_ptr in this context. Done means invalid returns are rejected and the relevant compiler tests pass.

Written by the indexing model from the issue text.