libstdc++: implement std::generator

libstdc++: implement std::generator

libstdc++-v3/ChangeLog:

	* include/Makefile.am: Install std/generator, bits/elements_of.h
	as freestanding.
	* include/Makefile.in: Regenerate.
	* include/bits/version.def: Add __cpp_lib_generator.
	* include/bits/version.h: Regenerate.
	* include/precompiled/stdc++.h: Include <generator>.
	* include/std/ranges: Include bits/elements_of.h
	* include/bits/elements_of.h: New file.
	* include/std/generator: New file.
	* testsuite/24_iterators/range_generators/01.cc: New test.
	* testsuite/24_iterators/range_generators/02.cc: New test.
	* testsuite/24_iterators/range_generators/copy.cc: New test.
	* testsuite/24_iterators/range_generators/except.cc: New test.
	* testsuite/24_iterators/range_generators/synopsis.cc: New test.
	* testsuite/24_iterators/range_generators/subrange.cc: New test.