Represent StringBuilder as ReadOnlySequence<char>

StringBuilderSegment.cs

public class StringBuilderSegment : ReadOnlySequenceSegment<char>
{
	public static ReadOnlySequence<char> FromStringBuilder(StringBuilder sb)
	{
		var enumerator = sb.GetChunks();
		if (!enumerator.MoveNext())
		{
			return ReadOnlySequence<char>.Empty;
		}

		var i = 0;
		var firstChunk = new StringBuilderSegment
		{
			RunningIndex = i,
			Memory = enumerator.Current,
		};
		var lastChunk = firstChunk;

		while (enumerator.MoveNext())
		{
			i += lastChunk.Memory.Length;
			var nextChunk = new StringBuilderSegment
			{
				RunningIndex = i,
				Memory = enumerator.Current,
			};
			lastChunk.Next = nextChunk;
			lastChunk = nextChunk;
		}
		
		return new ReadOnlySequence<char>(firstChunk, 0, lastChunk, lastChunk.Memory.Length);
	}
}