During b0e73277 we removed loaded_at_xy, but I kinda forgot that
it was a union with next_station. Now next_station wasn't copied
anymore, or checked in AreMergable.
(cherry picked from commit e6c02ebee6)
Over the years, things got reused and changed, making the current
names somewhat unclear in what they actually mean and do.
(cherry picked from commit 30172fc037)
Move them into a new GoodsEntryData struct referenced
using a std::unique_ptr from GoodsEntry.
The unique_ptr may be nullptr if the cargo list and flow stat map
are both empty (this is the case for unused cargoes).
This reduces GoodsEntry from 128 to 24 bytes,
and Station from 8680 to 2024 bytes,
(on Linux x86_64).
