Potential speedup for generic queries

[mlange-42]

Generic queries could be speed up by up to approx. 20% by:

• caching archetype column layout (updated when traversing to next archetype)
• using different layout types for mandatory vs. optional components, to avoid nil check branching

To keep the API clean, this would probably require to merge package generic into package ecs. Otherwise, methody not relevant for the user would need to be exposed.

Not sure if a maximum of 20% speedup if worth it.