libstdc++ PR 57272 Fancy pointer support in Hashtable

[François Dumont]

Here is my work in progress to use allocator pointer type. This type is 
used both as the node pointer and as the buckets pointer.

Rather than adapting _Local_iterator_base like _Node_iterator_base I 
prefer to just make it inherits from _Node_iterator_base. It simplifies 
its implementation and avoids to provided dedicated comparison operators.

Now I wonder if I need to consider Phil Bouchard comment regarding how 
node pointers are being passed, either by value or reference. I already 
chose to pass them as rvalue references in some occasions and even 
lvalue reference like in _M_bucket_index method. Do you think I need to 
continue this way ? Maybe I should use some conditional type, if raw 
pointer we pass by value and otherwise we pass by ref ?

François

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