Adding sentinel support to fmt::join().

[brevzin]

I agree that my contributions are licensed under the {fmt} license, and agree to future changes to the licensing.

This PR adds support to invoke fmt::join() on ranges that have a sentinel type that differs from its iterator type. This allows you to use fmt::join() on range-v3/C++20 ranges adapters. This shouldn't affect any existing code (whose sentinel is just the same as the iterator type).

[vitaut]

Thank you!

[puetzk]

This did technically break any code that instantiated fmt::arg_join<Iterator, char16_t> explicitly. I had one place that was doing this, because it wanted Char = char16_t, but fmt::join is only overloaded for fmt::string_view and fmt::wstring_view, and cannot be partially specialized in order to pull out Char from other options (in this case u16string_view).

The breakage could have been avoided if Sentinel had been introduced as the last template argument, and defaulted, i.e.

template <typename It, typename Char, typename Sentinel = It>
struct arg_join : detail::view {

OTOH, arg_join isn't in c++20 to be compatible with, and changing the template param order to that now would be a breaking change for anyone who has used it its is current form (since this change hit 7.0). Probably not worth trying to change it back...

[vitaut]

Oops, sorry. It is indeed too late to change it back.

[puetzk]

Not a problem, I had exactly one use, and it was trivial to fix, and you did label 7 as a major version (so potentially-breaking changes were not a surprise).