refactor: make the system's entities container a contiguous block of memory

[iMeaNz]

Objective

For now the entites in the systems are stored inside a std::set, this is not really efficient since std::set is not stored as contiguous block of memory like std::vector or std::array, and then it is not really cache-friendly.
We should change that, at first we tried std::vector but the search/reallocation cost makes so that adding/removing components or deleting entites is very inneficient.
We should try to implement our own container class with a cache-friendly and O(1) lookup implementation but also allowing fast search and allocation.

Acceptance Criteria

• 1. Implement a new container class
• 2. Add it to the system class

Dependencies

No response

Technical Details

No response

Type

Engine

[iMeaNz]

Done.
See #171 (comment) for more info.

Ready for review