Engine size affects power fix

While digging around in the code, I noticed that engine bigness is supposed to affect engine power - it's not just flavor text. The code for this was actually committed back in February, but was never called due to a typo in a bitflag mapping.

After fixing the typo, a few other minor issued cropped up, such as some vehicles being underpowered, which I've also addressed in this PR.

```Changes:
-fixed typo in bitflag map, which prevented engine size from affecting engine power
-fixed a couple power checks that weren't going through part_power()
-removed power ratings from gas engines in vehicle_parts.json, since they are no longer used
-adjusted size ranges for gas engines
-ambulance, armored_car, and policecar now use engine_v8 instead of engine_v6```