Define std::filesystem::path::format enum (P0430R2)

Define std::filesystem::path::format enum (P0430R2)

	* include/bits/fs_path.h (path::format): Define new enumeration type.
	(path(string_type&&), path<Source>(const Source&))
	(path<InputIterator>(InputIterator, InputIterator))
	(path<Source>(const Source&, const locale&))
	(path<InputIterator>(InputIterator, InputIterator, const locale&)):
	Add format parameter.
	* testsuite/27_io/filesystem/path/construct/format.cc: New test.

From-SVN: r254144