Formally Verify New Signature Requirements From #754

[nlordell]

Context / issue

#754 introduces new checks and restrictions on signature encoding that should be formally verified.

Proposed solution

Add a new formal verification rule that checks that for a valid set of signatures, you can only encoded it in a single bytes representation (length and order).

Alternatives

Don't formally verify it.

[nlordell]

Moving to the safe-modules repository