Minor simplifications for std::to_chars implementation

Minor simplifications for std::to_chars implementation

	* include/std/charconv (__detail::__to_chars_2_len): Use std::log2p1.
	(__detail::__to_chars_8_len): Remove.
	(__detail::__to_chars_8): Inline length calculation here.
	(__detail::__from_chars_binary): Use numeric_limits instead of
	CHAR_BIT.

From-SVN: r275313