Make deque iterator operators hidden friends

[François Dumont]

Here is a patch to reduce number of operators exposed at std namespace 
scope.

 Â Â Â  * include/bits/stl_deque.h
 Â Â Â  (_Deque_iterator<>::operator+(difference_type)): Make hidden friend.
 Â Â Â  (_Deque_iterator<>::operator-(difference_type)): Likewise.
 Â Â Â  (operator==(const _Deque_iterator<>&, const _Deque_iterator<>&)):
 Â Â Â  Likewise.
 Â Â Â  (operator!=(const _Deque_iterator<>&, const _Deque_iterator<>&)):
 Â Â Â  Likewise.
 Â Â Â  (operator<(const _Deque_iterator<>&, const _Deque_iterator<>&)):
 Â Â Â  Likewise.
 Â Â Â  (operator<=(const _Deque_iterator<>&, const _Deque_iterator<>&)):
 Â Â Â  Likewise.
 Â Â Â  (operator>(const _Deque_iterator<>&, const _Deque_iterator<>&)):
 Â Â Â  Likewise.
 Â Â Â  (operator>=(const _Deque_iterator<>&, const _Deque_iterator<>&)):
 Â Â Â  Likewise.

Tested under Linux x86_64 normal/debug modes.

Ok to commit ?

François

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