Improve type safeness of InitializedReport

[mendess]

InitializedReport has a Option<Vec<u8>> field which is only set by the helper, but this fact is known at compile time, it doesn't need to be encoded in a runtime option.

By replacing Option<Vec<u8>> with a generic that can either be () or WithPeerPrepShare we make it explicit which parts of the code are dealing with reports from a client and which parts are dealing with reports from a peer (leader).

In other words, we lift an invariant that was held by a comment

        // Set by the Helper.
        peer_prep_share: Option<Vec<u8>>

into one upheld by the compiler.

And finally instead of having on new constructor we have from_client and from_leader to make it obvious what the intent is when constructing, which in turn results in different sets of arguments being required (the former doesn't require a peer_prep_share where the latter does)

Other advantages of this change are:

• we no longer make clippy angry that our function has too many arguments
• we no longer need to ignore the Some case in leader code
• we no longer need to handle the None case in helper code
• we can no longer write tests that give the wrong idea of what the correct set of arguments are since it was incorrect to call new with the helper role and pass None to the peer_prep_share argument, but this test did it anyway

[armfazh]

is it possible to remove the Copy constraint?

[mendess]

Removed