- FrugalStructList's `ICollection<T>`-based constructor uses foreach to enumerate the contents of the collection.  If it's an `IList<T>`, we can instead index and avoid allocating the enumerator.
- Avoid multiple interface calls to `ICollection<T>.Count` in FrugalStructList's ctor
- Delete a dead ctor on `ArrayItemList<T>`.  That ctor was the only reason an array field may have been left null, so we can also remove subsequent null checks when accessing that array.
- Use Span/Array in ArrayItemList for Clear, Contains, IndexOf, ToArray, and CopyTo rather than open-coding them

Co-authored-by: Bradley Grainger <[email protected]>