Use contiguous_range when available

[user202729]

This provides further 20% speedup for vector<char> when used on top of #4679 (with the linked benchmark that has since then been rejected, but for the purpose of measuring the code runtime, it works well enough), for understandable reasons.

If one wants support for faster format to vector<char> before C++20, it's possible to specialize is_contiguous for vector, array etc. But I don't think it's worth it. For example, such template specialization would not work with custom string types.

I don't know if <ranges> is too heavy for base.h. There is contiguous_iterator instead in <iterator>. https://en.cppreference.com/w/cpp/iterator/contiguous_iterator.html

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Side note, thinking about it, since base.h defines is_contiguous to be always false, but format.h defines it to be true for string, wouldn't this lead to potential ODR?

• compilation unit A includes only base.h and use (something that uses) is_contiguous<string>.
• compilation unit B includes format.h and use (something that uses) is_contiguous<string>.

In practice, it's probably fine though.