Fix UB introduced by copying detail::FormatListN objects.

[dakep]

When copying a detail::FormatListN object, the m_formatterStore must also be copied, otherwise the pointer in the parent FormatList class are possibly invalid.

This addresses the segfault explained in #58. The issue arises when makeFormatList returns an object which holds a pointer to memory created for the temporary object on the stack.

[c42f]

Thanks for jumping in and fixing this, it's much appreciated (I still don't have an icc license, so pretty hard to test on my end).

I think those const_casts shouldn't be required. Other than that this looks great.

[dakep]

You are right! These const_casts are of course not required. 🤦‍♂

[c42f]

Looks good now, let's merge this.