Implement chunks

[samufi]

Chunks are contiguous blocks of memory (everything that is in one Archetype) and allow the same get features as an EntityAccessor. However, they return pointers to the beginning of the block so that efficient SIMD operations can be done. There should also be a get_as method that simply casts the pointer of the component but does an additional comptime size check. Lastly, an important attribute is size, so that a chunk can be efficiently processed in loops.

There should be an iterator that iterates over all chunks of a certain pattern.