Codechange: Make the road vehicle path cache behave more like a std::deque

[Kuhnovic]

Motivation / Problem

The Road Vehicle Path Cache is actually two std::deque's in one. That's a bit silly, but it is done to make saveload work. But the silliness has propaged into other parts of the code, where you basically have to push and pop items to both queues,

Description

I tried to keep the two-queues-in-one an implementation detail. From the caller's perspective it works just like a single std::deque. Sort of, because I only implemented the functions that were needed ;)

Limitations

Checklist for review

Some things are not automated, and forgotten often. This list is a reminder for the reviewers.

• The bug fix is important enough to be backported? (label: 'backport requested')
• This PR touches english.txt or translations? Check the guidelines
• This PR affects the save game format? (label 'savegame upgrade')
• This PR affects the GS/AI API? (label 'needs review: Script API')
  • ai_changelog.hpp, game_changelog.hpp need updating.
  • The compatibility wrappers (compat_*.nut) need updating.
• This PR affects the NewGRF API? (label 'needs review: NewGRF')
  • newgrf_debug_data.h may need updating.
  • PR must be added to API tracker

[PeterN]

It would actually be better to replace the two deque system. That was an oversight on my part that needlessly uses quite a lot of memory for each RoadVehicle object.

Some other container might be perfectly fine here too, I don't think it even needs to be a deque. With some reworking of how the node path is traversed, a simple vector should work as it only needs to be read from one end.