Packed/sparse representations

[Ralith]

It might be useful/fun to allow certain components to be stored per archetype in ways other than a flat array. For example, a bool component might be stored in a packed bitfield, and a bool component which is nearly always false might benefit from sparse encoding. This could be pursued with something like:

trait Container<T> { /* ... */ }

struct Array<T>(Box<[T]>);

impl<T> Container<T> for Array { /* ... */ }

trait Component<T: Container<Self> = Array<T>>: Send + Sync + Sized + 'static {}
impl<T: Send + Sync + 'static> Component<Array<T>> for T {}

This preserves the "It Just Works" behavior for typical cases, while allowing specialized implementations as needed. The proposed definition of Array would break up the currently monolithic Archetype allocations; in the event that this compromises performance, an allocator abstraction could be introduced to allow multiple containers to share a single memory region.

The most obviously useful nonstandard container, a bitfield, would require container-specific smart pointer types in place of references. This will be difficult to accomplish without GATs, so it's probably best to wait their arrival before developing this concept further.