New container: small_deque

[denzor200]

I propose the addition of boost::container::small_deque - a deque container that optimizes for small element counts by storing the first time appeared block in stack memory, similar to the existing boost::container::small_vector but maintaining deque semantics with efficient push/pop operations at both ends.

The standard std::deque typically performs heap allocations even for small numbers of elements, which can be suboptimal for deque instances which contain few elements.
Existing solutions like boost::container::small_vector provide stack optimization but lack efficient O(1) push/pop operations at both ends.

std::deque<int> d1 = {1, 2, 3};  // heap
boost::container::small_deque<int, 4> d2 = {1, 2, 3};  // Only stack
boost::container::small_deque<int, 4> d3 = {1, 2, 3, 4, 5};  // stack + heap (at least 20 bytes totaly used)
boost::container::small_vector<int, 4> v1 = {1, 2, 3};  // Only stack
boost::container::small_vector<int, 4> v2 = {1, 2, 3, 4, 5};  // Only heap (at least 36 bytes totally used)

void process(const boost::container::small_deque<int, 8>& items) {
    for (int item : items) {  // hot path cycle
        process_item(item);
    }
}