Move entity stuff to the ecs/entity directory

[RiscadoA]

Description

1. Moved entity manager to the ecs/entity directory.
2. Split Entity from manager.hpp into entity.hpp.
3. Replaced std::hash<Entity> specialization with EntityHash, which allows us to avoid including <functional> (yikes) in entity.hpp.

For the third point I had to add new template types to SerializationMap to allow specifying custom hash types.

Checklist

• Self-review changes.
• Evaluate impact on the documentation.

[github-actions]

PR Preview Action v1.4.4
🚀 Deployed preview to https://GameDevTecnico.github.io/cubos/pr-preview/pr-644/
on branch gh-pages at 2023-10-02 12:21 UTC

[codecov]

Codecov Report

Attention: 6 lines in your changes are missing coverage. Please review.

Comparison is base (77431c9) 36.52% compared to head (e9d6360) 36.56%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #644      +/-   ##
==========================================
+ Coverage   36.52%   36.56%   +0.04%     
==========================================
  Files          89       92       +3     
  Lines        6377     6378       +1     
==========================================
+ Hits         2329     2332       +3     
+ Misses       4048     4046       -2     

Files	Coverage Δ
.../include/cubos/core/data/old/serialization_map.hpp	69.23% <ø> (ø)
core/include/cubos/core/ecs/blueprint.hpp	94.73% <100.00%> (ø)
core/include/cubos/core/ecs/commands.hpp	95.65% <ø> (ø)
core/include/cubos/core/ecs/entity/entity.hpp	100.00% <100.00%> (ø)
core/include/cubos/core/ecs/entity/manager.hpp	100.00% <ø> (ø)
core/include/cubos/core/ecs/storage.hpp	72.72% <ø> (ø)
core/include/cubos/core/ecs/world.hpp	88.88% <ø> (ø)
core/src/cubos/core/ecs/blueprint.cpp	100.00% <100.00%> (ø)
core/src/cubos/core/ecs/commands.cpp	97.29% <100.00%> (+0.03%)	⬆️
core/src/cubos/core/ecs/entity/hash.cpp	100.00% <100.00%> (ø)
... and 2 more

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[github-actions]

⚠️ clang-tidy found issue(s) with the introduced code (1/1)

No clang-tidy warnings found so I assume my comments were addressed