Merging format

[barcharcraz]

• adds an implementation of <format>
• Resolves P0645R10 <format> Text Formatting #30.
• Resolves <format>: Avoid dependency on <variant> #1826.

[StephanTLavavej]

• Include more headers.
• In P0645R10_text_formatting_formatting, rename integral to Integral to avoid shadowing the Standard concept.
• Representation change: Change _Align and _Sign's underlying type to uint8_t. _Align is stored as a data member, so this matters.
• static_assert that _Basic_format_arg_type::_Custom_type is strictly less than 16, because it must fit in a 4-bit bitfield.
• Style: Add newlines.
• Bugfix: Add missing typedefs format_parse_context and wformat_parse_context.
• Representation change (sort of): basic_format_arg::handle should store a __cdecl function pointer. This will make it immune to the default calling convention.
• Bugfix (maybe?): basic_format_arg::handle::format() is const in the Standard.
• Bugfix: visit_format_arg() must return decltype(auto) as depicted by the Standard.
• Style: constexpr explicit is conventional.
• Add _NODISCARD to _Find_encoded().
• Style: Use braces to construct temporaries.
• Style: In _Format_arg_store_packed_index, avoid shadowing between the constructor parameter and data member, and remove an unnecessary static_cast (they're both size_t).
• Representation change: Add alignas(_Index_type) to _Format_arg_store::_Storage.
  • Also fix the comment; _Index_length is the number of bytes needed to store _Num_args elements of type _Format_arg_store_packed_index.
• Improve comments.
  • Fix typo, _OpenCurl should be _OpeningCurl.
  • Add a WP citation.
  • Overhaul the _Format_min_buffer_length comment, fixing its math.
  • "hex number" is slightly unclear; other comments in the STL (especially <charconv>) say "hexit" (for "hexadecimal digit").
  • I couldn't understand what "(with overloaded)" intended to say, so I dropped it.
  • Capitalize "ASCII".
• Style: Adjust spacing and indentation of requires clauses. A space goes after requires when it doesn't look like a function signature.
• Style: When declaring functions, don't mark value parameters as const.
• Use isfinite for simplicity.
• Style: Use _Nx for N instead of _Nty (it's not a type, or a number of types).
• Big rename: _Align to _Fmt_align.
• Big rename: _Sign to _Fmt_sign.

[StephanTLavavej]

I've pushed a merge with main to resolve a conflict with #1832 in <xutility>: that PR added _Rewrap_iterator above _Convertible_from which this PR is moving. I resolved it by leaving _Rewrap_iterator in place (now immediately above in_in_result) so only _Convertible_from is moving. This is fine because nothing in <xutility> itself needs _Rewrap_iterator.

[StephanTLavavej]

Thanks to everyone who worked on implementing this major feature! 🎉 😻 🚀

[Sixshaman]

Thank you everyone! I've been waiting for that

[CaseyCarter]

Belated approval: everything looks acceptable to me (which is no great surprise, since I've reviewed most of this already, and worked on some of it).