Simplify _Rb_tree instantiation

[François Dumont]

Hi

As we are at working on associative containers I'd like to propose this 
last patch to remove the copy constructible constraint on the _Compare 
functor when it is supposed to be default constructed.

This way the _Compare is built directly at its final place.

 Â Â Â  * include/bits/stl_tree.h (_Rb_tree_impl(_Node_allocator&&)): New.
 Â Â Â  (_Rb_tree(const allocator_type&)): Use latter.
 Â Â Â  * include/bits/stl_map.h (map(const allocator_type&)): Likewise.
 Â Â Â  (map(initializer_list<value_type>, const allocator_type&)): Likewise.
 Â Â Â  (map(_InputIterator, _InputIterator, const allocator_type&)): Likewise.
 Â Â Â  * include/bits/stl_multimap.h
 Â Â Â  (multimap(const allocator_type&)): Likewise.
 Â Â Â  (multimap(initializer_list<value_type>, const allocator_type&)):
 Â Â Â  Likewise.
 Â Â Â  (multimap(_InputIterator, _InputIterator, const allocator_type&)):
 Â Â Â  Likewise.
 Â Â Â  * include/bits/stl_set.h (set(const allocator_type&)): Likewise.
 Â Â Â  (set(initializer_list<value_type>, const allocator_type&)): Likewise.
 Â Â Â  (set(_InputIterator, _InputIterator, const allocator_type&)): Likewise.
 Â Â Â  * include/bits/stl_multiset.h
 Â Â Â  (multiset(const allocator_type&)): Likewise.
 Â Â Â  (multiset(initializer_list<value_type>, const allocator_type&)):
 Â Â Â  Likewise.
 Â Â Â  (multiset(_InputIterator, _InputIterator, const allocator_type&)):
 Â Â Â  Likewise.

Tested under linux x86_64.

Ok to commit ?

François

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