PR libstdc++/68222 Hide safe iterator operators

[François Dumont]

Here is the new proposal. It is indeed possible to keep _Safe_iterator 
and just add a _Category template parameter to it.

I introduce a friend declaration to access container _Base nested 
typedef from the safe iterator.

I review the safe const_iterator constructor from safe iterator. I now 
check if we are in the a const_iterator context so that compilers don't 
even try to consider this constructor when we are in an iterator context.

I adapted _Safe_local_iterator the same way to keep consistency with 
_Safe_iterator and because I find the new design cleaner. It also fixes 
the same problem I fixed on _Safe_iterator when checking it iterator has 
been initialized using _MutableIterator() rather than _Iterator().

I stop overloading __get_distance for safe iterators or safe local 
iterators, it was useless. I prefer to introduce similar functions as 
members. Same for __get_distance_from_begin or __get_distance_to_end, 
and I move code in safe_iterator.tcc.

Tested under Linux x86_64 debug mode.

Ok to commit ?

François
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