FOO::csv_line

[yakra]

Many (all?) of the csv_line functions could probably become voids that insert multiple C strings directly into the ofstream.
HighwaySegment::csv_line for example has 8 concatenation + operators. All this copying & re-copying gets expensive.

While in there, also check out TravelMapping#324

[yakra]

TravelMapping#505 was put on hold, but a rebased copy of 597682e should re-appear shortly.
That takes care of Route & ConnectedRoute. Instead of a void, inlined the insertions right into sql_file.cpp.
For the sake of Getting On With It, I'll assume there's no point in doing void functions (8 more lines of code overall) just to get rid of some dereferencing there. Creating references didn't help speed, I'll assume such a "dereference" would be created under the hood by such a function. Either the multiple dereferences I have now don't hurt, or it all gets optimized away by the compiler.

That leaves Waypoint & HighwaySegment, which can be handled as part of the broader-scope #213.