This refactors the ship registry and the OSWPT struct to store data as indexes rather than pointers. This is a prerequisite for making these collections dynamic in the future. See also #5800.
The Star Fox barrel roll effect is achieved using a script which spawns a new ship. The script sets no-collide on this spawned ship, but it also tries to set the ship flag for all future ships -- and future ships require a parse object, which doesn't exist for a created ship. This patches the FALLTHROUGH in the sexp code to account for that possibility.
Separately, there are other ways a parse object pointer could become null through unusual code states, so this adds an INVALID status and several additional checks to defend against this crash in other circumstances.
When iterating through `Ship_obj_list`, check the `Should_be_dead` flag before using the object. This fixes#4990 and adds defense-in-depth checks to guard against similar errors elsewhere.
I considered moving the `Ship_obj_list` removal from `obj_delete` to `ship_cleanup`, but decided it would be prudent to avoid fundamental changes to lifecycle management.
Goober5000