Add `range_formatter`

[JMazurkiewicz]

• Remove used by pair, tuple, (...) comment from _Fill_align_and_width_specs - it no longer serves its purpose.
• Make _Fill_align_and_width_specs_setter<T>::_Specs member variable protected, so that we can create derived _Range_specs_setter.
• Implement range_formatter:
  • range_formatter::format can be used at runtime only if format context's output iterator is back_insert_iterator<_Fmt_buffer<T>>. Creation of nested basic_format_context requires copying format args from old context<X> to context<Y> and X and Y have to be the same type to do this. This should not be a problem anyway, since basic_format_context can be constructed only by the standard library.
  • Towards #2919.