Generic factory std::map support

[ShnitzelX2]

Summary

Infrastructure "generic factory std::map support"

Purpose of change

This is a major obstacle on the long road to completing #36851. We have a ton of weighted list (or functional equivalent) fields for items that are read manually.

Describe the solution

A handler for std::map in generic_factory allows two new readers to be added:

• pair_reader, which loads simple type pairs into a map
• weighted_string_id_reader, which loads pairs of string_ids and floats/ints in a variety of shorthanded formats

Describe alternatives you've considered

Anything more complex than a map should be a separately loaded object, string_id-typed if necessary

Testing

I didn't include any implementations of these readers, but I did test with two fields from Item_factory::load( islot_comestible &slot, ...) like so:

For the weighted_string_id_reader, I had to reformat the contamination field, but it otherwise worked fine with every format (array of arrays, string_id array, single string_id), and I verified that the fields loaded correctly in the debugger. Drinking murky water still gave food poisoning.

For the pair_reader, modding a retool_ar15_223rem_short onto an AR-15 worked the same as without the pair_reader (tested with an earlier CDDA build).

Additional context

I've done a lot of studying of item_factory and generic_factory code, so I hope to start working on migrations from the former to the latter soon.

[RenechCDDA]

This is some important infrastructure work, thank you for doing it.

nutrients::deserialize contains an existing (de)serialization of a map, if you want a live example to work with. Also the map normally contains a union, for extra stress testing :)