This refactors the object_h struct to store objects as indexes rather than pointers. This is a prerequisite for making the collection of objects dynamic in the future. See also #6100.
Follow-up to #5800 and #5829.
Adds gravity! Applies on a per-mission basis, and weapons, ships, debris, and asteroids now have a configurable "gravity const" which multiplies the effect of gravity on them, by default everything except debris is 0 (meaning no effect even when the mission has gravity). AI and the lead indicator will also lead for ballistic trajectories as well, given a gravity-affected weapon. No FRED support or gravity direction control yet, to come in later PRs.
Properly whacks all ships that are docked together when any are whacked, makes everything more consistent by calling ship_apply_whack instead of physics_apply_whack, whacking in general now unrotates its hit pos to get a world pos like physics whack was expecting and adds a few matrix helper functions.
Many physics values are exposed to scripting, including rotational damp. This PR exposes the linear damp to scripting. Note that in the code the linear damp is called `side_slip_time_const`.
Goober5000