Perf: stackalloc on short inputs to StringPool.GetOrAdd(ReadOnlySpan<byte>, Encoding) #699

public unsafe string GetOrAdd(ReadOnlySpan<byte> span, Encoding encoding)
{
    if (span.IsEmpty)
    {
        return string.Empty;
    }

+   if (maxLength <= 128)
+   {
+       Span<char> buffer = stackalloc char[maxLength];

+       fixed (byte* source = span)
+       fixed (char* destination = &buffer.DangerousGetReference())
+       {
+           int effectiveLength = encoding.GetChars(source, span.Length, destination, maxLength);
+
+           return GetOrAdd(new ReadOnlySpan<char>(destination, effectiveLength));
+       }
+   }
+   else
+   {
        using SpanOwner<char> buffer = SpanOwner<char>.Allocate(maxLength);

        fixed (byte* source = span)
        fixed (char* destination = &buffer.DangerousGetReference())
        {
            int effectiveLength = encoding.GetChars(source, span.Length, destination, maxLength);

            return GetOrAdd(new ReadOnlySpan<char>(destination, effectiveLength));
        }
+   }
}