libstdc++: Add comparison operators to types from Numerics clause

libstdc++: Add comparison operators to types from Numerics clause

Some more C++20 changes from P1614R2, "The Mothership has Landed".

	* include/bits/slice_array.h (operator==(const slice&, const slice&)):
	Define for C++20.
	* include/std/complex (operator==(const T&, const complex<T>&))
	(operator!=(const complex<T>&, const complex<T>&))
	(operator!=(const complex<T>&, const T&))
	(operator!=(const T&, const complex<T>&)): Do not declare for C++20.
	* testsuite/26_numerics/slice/compare.cc: New test.