[Proposal discussion]: Allow arrays as CollectionBuilder Create parameter type

[jnm2]

• Proposal: Proposal: Allow arrays as CollectionBuilder Create parameter type #9922
• Issue: [Proposal]: Allow arrays as CollectionBuilder Create parameter type #9923

Summary

Types may now opt into support for collection expressions by using CollectionBuilderAttribute with a Create method that takes an array rather than a readonly span of the elements.

This will enable types such as ReadOnlyMemory<T> and ArraySegment<T> to support collection expressions without incurring the performance penalty of copying from a span to a second array to use as their backing storage.

[CollectionBuilder(typeof(ReadOnlyMemory), nameof(ReadOnlyMemory.Create))]
public readonly struct ReadOnlyMemory<T> : IEquatable<ReadOnlyMemory<T>> { ... }

// (Or MemoryMarshal, or some other holder)
public static class ReadOnlyMemory
{
    // Now allowed: T[] instead of ReadOnlySpan<T> parameter
    public static ReadOnlyMemory<T> Create<T>(T[] array) => array;
}

(Note: for Memory<T> and ReadOnlyMemory<T> to be used with collection expressions, the already-approved Relax requirement that type be enumerable to participate in collection expressions feature must also be implemented.)

[jnm2]

This discussion is for the proposed concrete solution which simultaneously addresses the following discussions:

• `ArraySegment<T>` doesn't work well with collection expressions. #8164
• Add collection literal support for `Memory<T>` and `ReadOnlyMemory<T>`. #8842
• Allow Memory<T> to be target of collection expressions #9495