There are still some things left to do:
* Implement despawn
* Maybe shrink SlotVector if there's a lot more free slots than occupied slots
* Implement iteration over entities

* reworked some resource loading and mod init stuff
* texetures are now configured at runtime via .toml files
* moved from SRF to messagepack
* changed folder structures
* added the fmt library (for later)

This should've been multiple commits, but hey, it's late.