std::forward_list optim for always equal allocator

[François Dumont]

Hi

     Here is the patch to implement the always equal alloc optimization 
for forward_list. With this version there is no abi issue.

     I also prefer to implement the _Fwd_list_node_base move operator 
for consistency with the move constructor and used it where applicable.


     * include/bits/forward_list.h
     (_Fwd_list_node_base& operator=(_Fwd_list_node_base&&)): Implement.
     (_Fwd_list_impl(_Fwd_list_impl&&, _Node_alloc_type&&)): New.
     (_Fwd_list_base(_Fwd_list_base&&, _Node_alloc_type&&, std::true_type)):
     New, use latter.
     (forward_list(forward_list&&, _Node_alloc_type&&, std::false_type)):
     New.
     (forward_list(forward_list&&, _Node_alloc_type&&, std::true_type)):
     New.
     (forward_list(forward_list&&, const _Alloc&)): Adapt to use latters.
     * include/bits/forward_list.tcc
     (_Fwd_list_base(_Fwd_list_base&&, _Node_alloc_type&&)): Adapt to use
     _M_impl._M_head move assignment.
     (forward_list<>::merge(forward_list<>&&, _Comp)): Likewise.

Tested under Linux x86_64, ok to commit ?

François

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