<deque>: many internal operations could use unchecked iterators

[CaseyCarter]

deque is inconsistent about using checked vs. unchecked iterators for internal operations where checking is not needed. emplace, for example:

STL/stl/inc/deque

Lines 819 to 835 in ef62d3f

	template <class... _Valty>
	iterator emplace(const_iterator _Where, _Valty&&... _Val) {
	const auto _Off = static_cast<size_type>(_Where - begin());
	#if _ITERATOR_DEBUG_LEVEL == 2
	_STL_VERIFY(_Off <= _Mysize(), "deque emplace iterator outside range");
	#endif // _ITERATOR_DEBUG_LEVEL == 2
	if (_Off <= _Mysize() / 2) { // closer to front, push to front then rotate
	emplace_front(_STD forward<_Valty>(_Val)...);
	_STD rotate(begin(), _Next_iter(begin()), begin() + static_cast<difference_type>(1 + _Off));
	} else { // closer to back, push to back then rotate
	emplace_back(_STD forward<_Valty>(_Val)...);
	_STD rotate(begin() + static_cast<difference_type>(_Off), _Prev_iter(end()), end());
	}
	return begin() + static_cast<difference_type>(_Off);
	}

passes checked iterators to rotate which could as well be unchecked - we know that the ranges we're forming are valid. We need to audit the file for such occurrences and change them to use unchecked iterators (returned from _Unchecked_begin and _Unchecked_end) where possible.