Use Span to fill List<T> in more ToList scenarios

[brantburnett]

Adds span-based filling of List<T> during calls to ToList using CollectionsMarshal.SetCount to the following additional scenarios:

• ToList of a Union or Distinct
• ToList of a Lookup<TKey, TElement> including cases within grouping
• ToList of a Select projection against an IPartition (i.e. sorted)
• ToList of an OrderedEnumerable returning a single element (i.e. Skip/Take applied to a sorted list for a single result)
• ToList of a DefaultIfEmpty where the source is empty and an IIListProvider
• ToList of Append and Prepend scenarios

Secondary improvements:

• ToArray of an empty Lookup<TKey, TElement> now uses Array.Empty

Fixes #80760

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Issue Details

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Adds span-based filling of List<T> during calls to ToList using CollectionsMarshal.SetCount to the following additional scenarios:

• ToList of a Union or Distinct
• ToList of a Lookup<TKey, TElement> including cases within grouping
• ToList of a Select projection against an IPartition (i.e. sorted)
• ToList of an OrderedEnumerable returning a single element (i.e. Skip/Take applied to a sorted list for a single result)
• ToList of a DefaultIfEmpty where the source is empty and an IIListProvider
• ToList of Append and Prepend scenarios

Secondary improvements:

• ToArray of an empty Lookup<TKey, TElement> now uses Array.Empty

Fixes #80760

Author:	brantburnett
Assignees:	-
Labels:	area-System.Collections
Milestone:	-