Rework of vehicle repair/install/remove times.

Goals:
 - Make it cheaper/faster to do small repairs
 - Doing repairs back-to-back is slightly faster
 - Make time taken depend on skill, difficulty of part, and how much it's damaged
 - Doing lots of small repairs to bring vehicle up to perfect status should be a lot faster, doing complex things with little skill should be slower

Added "last_repair_turn" to vehicle so it can figure out if repairs are back-to-back or not.
Removed a redundant refresh_all(), added one when repairing because clock wasn't updating properly.
Timing values are not extensively tested, could probably be tuned later.
It can now take up to 2 hours for really complex tasks (improves with skill down to around the old value of 20 mins), but small repairs can take as little as a minute. Should on average be maybe twice as fast overall if you have insane mechanical skill, but will be a lot slower at low skill.
Reduced resource cost and skill gain in addition to time for small repairs.