☂️ Port DDA performance Optmization PRs

[scarf005]

Only some of @Aivean's NPC performance optimizations were ported to BN. This umbrella issue tracks them.

Flag optimizations

Flag & Big optimizations

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1. Ported items flags optimization from DDA #3354
   JSON src tests +3
   
2. port Speedup items processing by caching and skipping repeated checks of item tags CleverRaven/Cataclysm-DDA#45132
3. port Replace int_id<json_flag> with string_id<json_flag> (housekeeping) CleverRaven/Cataclysm-DDA#45277
4. port Replace int_id<json_flag> with string_id<json_flag> (housekeeping) CleverRaven/Cataclysm-DDA#45277
5. port Mass-rename flag_str_id to flag_id (housekeeping) CleverRaven/Cataclysm-DDA#45516
6. port Add reachability cache/filter that can reject bresenham-based LOS checks in O(1) time CleverRaven/Cataclysm-DDA#45346
7. port Performance: Cache NPC goal CleverRaven/Cataclysm-DDA#54312

NPC and other optimizations

Tasks

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1. port speedup npc::get_monster_faction #3074
   +0
   
2. port use vector::reserve to speedup squares_closer_to #3075
   +0
   
3. port return result of Creature::get_body by const reference #3076
   +0
   
4. port Speedup apply_mut_encumbrance by calculating exclusive_flag_coverage on demand CleverRaven/Cataclysm-DDA#45461
5. perf: speedup get_clothing_mod_val by caching get_var key #3078
   +0
6. port Replace dynamic instantiation of bodypart ids with constants CleverRaven/Cataclysm-DDA#45426
7. port Use field_cache in map::get_field to speedup fire check in npc::assess_danger #3079
   +0
   
8. port Reduce overhead of item::covers CleverRaven/Cataclysm-DDA#45026

Considerations

• Ports should not cause significant merge conflict for following PRs:
  • Lua via sol2 #2216
  • refactor!: give items identity #2250

#2250 may render some of those PRs obsolete, will have to check if it's the case.
also these PRs may have been already ported to BN, they also needs to be checked.

maybe just port https://github.com/CleverRaven/Cataclysm-DDA/pulls?q=is%3Apr+author%3AAivean+sort%3Acreated-asc+created%3A>2020-05-03 one by one?

[scarf005]

cc @Coolthulhu @olanti-p @joveeater