Create crates in order (for Engine V2)

[mr-smidge]

New method create_..._after() provides the ability to maintain a sorted list of crates. Pseudo-code as follows could achieve this:

djinterop::crate create_sorted(djinterop::database& db, const std::string& name)
{
  for (auto&& crate : db.root_crates())  // Assume already sorted
  {
    if (name > crate.name())
    {
      return db.create_root_crate_after(name, crate);
    }
  }

  return db.create_root_crate(name);
}

At present this is only implemented for Engine V2, which makes the process easy via its DB triggers. The DB schema in Engine V1 would require a little extra work that is not high priority now, given that Engine 1.x is not readily used any longer.

Fixes #117.