Eliminate double copying in vformat_to_n

[Roman-Koshelev]

format_to_n () may not be slower than format_to(). To do this, we will format immediately into the resulting buffer (until it ends)

[vitaut]

Thanks for the PR. I think this should be merged into the main iterator_buffer template since it's almost identical.

[Roman-Koshelev]

At first glance, yes, that's what I wrote. But it seemed to me that it is easier to take out into a separate class than to fence all in one

[vitaut]

OK, let's keep it as a separate specialization.

[vitaut]

Thank you!