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.

Tested powerpc64le-linux, committed to master.