Conform std::iterator_traits<fmt::appender> to [iterator.traits]/1

[CaseyCarter]

In addition, the types

iterator_traits<I>::pointer
iterator_traits<I>::reference

shall be defined as the iterator’s pointer and reference types; that is, for an iterator object a of class type, the same type as decltype(a.operator->()) and decltype(*a), respectively. The type iterator_traits<I>::pointer shall be void for an iterator of class type I that does not support operator->. Additionally, in the case of an output iterator, the types

iterator_traits<I>::value_type
iterator_traits<I>::difference_type
iterator_traits<I>::reference

may be defined as void.

Also removes the unnecessary member types from basic_appender.

[CaseyCarter]

Some C++20 tests that my naive local build didn't enable apparently want to use basic_appender without the iterator_traits specialization. I'll need to bring back at least the nested difference_type.

EDIT: Not exactly. The issue here was that I forgot the "void difference_type for output iterators" rule is only preserved for backwards compatibility with legacy iterators and all of the C++20 iterator concepts reject such an iterator.

[CaseyCarter]

Ok, this now compiles FMT_PEDANTIC C++20 on at least MSVC and GCC. Sorry for the noisy PR =)

[vitaut]

Thank you!