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).
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