libstdc++: Implement constexpr std::to_chars for C++23 (P2291R3)

libstdc++: Implement constexpr std::to_chars for C++23 (P2291R3)

Some of the helper functions use static constexpr local variables, which
is not permitted in a core constant expression. Removing the 'static'
seems to have negligible performance effect for __to_chars and
__to_chars_16. For __from_chars_alnum_to_val removing the 'static'
causes a significant performance impact for base 36 conversions. Use a
consteval lambda instead.

libstdc++-v3/ChangeLog:

	* include/bits/charconv.h (__to_chars_10_impl): Add constexpr
	for C++23. Remove 'static' from array.
	* include/std/charconv (__cpp_lib_constexpr_charconv): Define.
	(__to_chars, __to_chars_16): Remove 'static' from array, add
	constexpr.
	(__to_chars_10, __to_chars_8, __to_chars_2, __to_chars_i)
	(to_chars, __raise_and_add, __from_chars_pow2_base)
	(__from_chars_alnum, from_chars): Add constexpr.
	(__from_chars_alnum_to_val): Avoid local static during constant
	evaluation. Add constexpr.
	* include/std/version (__cpp_lib_constexpr_charconv): Define.
	* testsuite/20_util/from_chars/constexpr.cc: New test.
	* testsuite/20_util/to_chars/constexpr.cc: New test.
	* testsuite/20_util/to_chars/version.cc: New test.