Follow-up from "Implement handling of deposit-values in SCORU": Resolve circular dep due to Bond_id in alpha_context

This MR introduces Sc_rollup_repr in alpha_context.mli before the "official/recommanded" Sc_rollup wrapper just to avoid a circular dependency with Bond_id.t which references Sc_rollup_repr.t

-> Find a better way to break the dep (see details in the related thread below)

The following discussion from !5398 (merged) should be addressed:

• @iguerNL started a discussion: (+11 comments)

  Couldn't the circular dep be broken if the definition of Sc_rollup is done before Bond_id. It's maybe a lot of work, But I think it's better.