Change: Use bitmap for free unit ID generation. (#12165)

This improves performance of finding the next free unit number for a vehicle.

Based loosely on pool's used slot bitmap.

src/vehicle_base.h

@@ -755,7 +755,7 @@ public:
 	 * and that shall not be resetted for the new vehicle.
 	 * @param src The old vehicle
 	 */
-	inline void CopyVehicleConfigAndStatistics(const Vehicle *src)
+	inline void CopyVehicleConfigAndStatistics(Vehicle *src)
 	{
 		this->CopyConsistPropertiesFrom(src);
 
@@ -766,6 +766,8 @@ public:
 
 		this->profit_this_year = src->profit_this_year;
 		this->profit_last_year = src->profit_last_year;
+
+		src->unitnumber = 0;
 	}
 
 
@@ -1270,19 +1272,6 @@ struct SpecializedVehicle : public Vehicle {
 	static Pool::IterateWrapper<T> Iterate(size_t from = 0) { return Pool::IterateWrapper<T>(from); }
 };
 
-/** Generates sequence of free UnitID numbers */
-struct FreeUnitIDGenerator {
-	bool *cache;  ///< array of occupied unit id numbers
-	UnitID maxid; ///< maximum ID at the moment of constructor call
-	UnitID curid; ///< last ID returned; 0 if none
-
-	FreeUnitIDGenerator(VehicleType type, CompanyID owner);
-	UnitID NextID();
-
-	/** Releases allocated memory */
-	~FreeUnitIDGenerator() { free(this->cache); }
-};
-
 /** Sentinel for an invalid coordinate. */
 static const int32_t INVALID_COORD = 0x7fffffff;