More precise slewing

This patch replaces the internals of the RemoteTech flight computer with the corresponding MechJeb code, including their PID controller and a manual calculation of the ship's rotational inertia. I've tested it briefly with both a light and a heavy ship, and it appears to slew to any direction with no wobbling or tumbling. This patch does **not** include the RCS fix in RemoteTechnologiesGroup/RemoteTech#97. At the moment, ships slewing via RCS never quite settle down, likely because they're overcompensating for small errors.

In addition to ripping out the guts of `FlightCore`, this patch adds some vessel-state code to `FlightComputer`, and I'd like feedback from someone familiar with the code on whether that's the most appropriate place for it.

Test version is up at https://github.com/Starstrider42/RemoteTech/releases/tag/slew-fix-1 if you'd like to try it on a bigger variety of ships.