proto/scoru: simplify proof type even more

Context

In this MR purpose is only to simplify the inbox proof type to minimal. With the introduction of sol/eol there is always an inbox for each level. Now both single_level and next_level are of a same type, an inclusion proof and a message proof.

The function verifies that for a snapshot and first_after(l, i) the proof is valid and returns the payload with the following semantics:

• Before:

(level_tree l) included_in snapshot 
AND 
  | Single_level
         ( i + 1 < max_index_of (level_tree l)                        -> Some (message_at (level_tree l) ~index:(i + 1) )
      OR ( i + 1 > max_index_of (snapshot) && level_tree l = snapshot -> None ) 
  | Next_level
         ( message_at (level_tree l) ~index:(i + 1) = None ) -> Some (sol_at (l + 1) ~index:0)

With the simplified inline version we have :

    (level_tree l) included_in snapshot 
AND (
     ( i + 1 < max_index_of (level_tree l)                            -> Some (message_at (level_tree l) ~index:(i + 1)
  OR ( i + 1 > max_index_of (level_tree l) && level_tree l = snapshot -> None)
  OR ( i + 1 > max_index_of (level_tree l) && level_tree l < snapshot -> Some (sol_at (l + 1) ~index:0))

Manually testing the MR

Checklist

• Document the interface of any function added or modified (see the coding guidelines)

• Document any change to the user interface, including configuration parameters (see node configuration)

• Provide automatic testing (see the testing guide).

• For new features and bug fixes, add an item in the appropriate changelog (docs/protocols/alpha.rst for the protocol and the environment, CHANGES.rst at the root of the repository for everything else).

• Select suitable reviewers using the Reviewers field below.

• Select as Assignee the next person who should take action on that MR