Converts Ship_info from a ship_info[] to an SCP_vector<ship_info>. ship_info now has proper constructors, assignment operators, and a destructor. Num_ship_classes removed; various references to Num_ship_classes and/or MAX_SHIP_CLASSES (which remains for various other structures which need it) changed to Ship_info.size() instead.
This doesn't make the ship class limit dynamic, but it is a preparatory step to get there.