Split `mithril-common` crate - Preliminary work

[jpraynaud]

Why

We have worked on a split of the mithril-common crate in the issue #2175. We want to implement the first round of modifications that are mandatory to the rest of the split.

What

Implement the preliminary modifications for the mithril-common split

How

• Removal of the random feature: (as described in Design mithril-common split & re-organization in repository #2175 (comment))
• Removal of some circular dependencies: (as described in Design mithril-common split & re-organization in repository #2175 (comment))
  • entities <-> messages
  • entities <-> protocol
  • entities <-> signable_builder