Arche v0.15.0 released

[mlange-42]

Arche v0.15.0 features optimizations that vastly speed up the creation of huge numbers (millions) of entities. Namely, all memory is grown exponentially now, rather than linearly. This also causes a breaking change, as the former "capacity increments" turned into just initial capacities.

Further, the README was revised and the ECS competition benchmarks were extended and moved to the go-ecs-benchmarks repository.

See the CHANGELOG or the release for a list of all changes.