Bind to std::equal plumbing in ranges::equal

[François Dumont]

I started to work on ranges::equal to find out if what I am trying to do 
is totally silly.

With this patch ranges::equal is in pare with std::equal specializations 
that is to say that it correctly deals with Debug mode or std::deque 
iterators.

Once below patch is in:

https://gcc.gnu.org/ml/libstdc++/2019-12/msg00032.html

We will even be able to call std::__equal_aux1 directly using 
__niter_base to get rid of the Debug safe iterator layer. And even in 
this case get rid of the branch __use_memcmp and leave it to __equal_aux1.

In addition to testsuite I checked running gdb that it does the right thing.

Ok to commit ?

     libstdc++ Leverage on std::equal plumbing in ranges::equal.

     Benefit from the std::equal plumbing to correctly deal with
     _GLIBCXX_DEBUG mode and std::deque iterators.

             * include/bits/ranges_algobase.h (__equal_fn::operator()):
             Review conditions to call std::__equal_aux.
             * testsuite/25_algorithms/equal/constrained.cc (test04): New.

François


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