Better handling of enum comparisons

[danth]

What

Comparing enums with <, >, =, != is not ideal, because adding new entries to the enum could break the original assumptions.

Originally suggested at #392 (comment)

Acceptance Criteria (DoD)

• No comparisons using < or >
• Minimal comparisons using = or !=

How

In most cases, a switch without a default is much more robust.

For example:

FileExistenceState convertFileExistenceState(fb::FileExistenceState fb_state)
{
  switch (fb_state)
  {
    case fb::FileExistenceState::Deleted:
      return FileExistenceState::Deleted;
    case fb::FileExistenceState::Exists:
      return FileExistenceState::Exists;
  }

  SCORE_LANGUAGE_FUTURECPP_UNREACHABLE_MESSAGE("Invalid fb::FileExistenceState");
}

Not having a default triggers a compiler warning when a new entry is added to the enum, prompting the developer to consider the desired behaviour and add a new case.

Because it is possible (using casts) to create a value which does not correspond to any enum entry, SCORE_LANGUAGE_FUTURECPP_UNREACHABLE is needed to satisfy the compiler that the function will return in all cases. This will crash the program if an invalid value is seen.