Yaws Slabbable trait

Put a slab into it or Yaws oriented trait requirement for slab implementation.

lib.rs:

Slabbable is the trait for storage slab-slotmaps for the purpose of keeping the underlying request data alive when kernel is either modifying or keeping a reference of the said data through raw pointers.

This trait is opinionated into purpose of network server proramming that may have high and low load periods with ramp-up and downs.

The underlying implementation does not need to be Sync or Send and typically should live within one thread only requiring no synchronisation / atomics.

The user of the slab-slotmap must guarantee that the owned slotmap itself is not dropped whilst guaranteeing the items inside are not moving or dropped.

Required Guarantees

The slab-slotmap implementing this trait must:

1. Keep the memory addresses stable as-in self-referential structs
2. Provide free slot and upon freeing the slot must be re-usable
3. Lookable key by usize that can be copy-referenced without pointer access
4. Should not leak memory beyond the fixed capacity max.

Desired Properties

A. Fast random access via key that is usize B. Occupying takes a sequential usize id C. Not re-using sequential usize id until it is recycled at usize::MAX D; Ability to free-up memory e.g. in acses of ramp-up / down high/low loads. E. Minimal memory usage for free slots