Use real_allocator to find allocator when rebinding

[ecatmur]

After 83bb62f the allocator parameter of
e.g. small_vector can be void. Use real_allocator to convert this to the
actual allocator used by the container.

example (works up to 1.69, breaks in 1.70):

#include <boost/container/flat_map.hpp>
#include <boost/container/small_vector.hpp>
boost::container::flat_map<int, int, std::less<>,
    boost::container::small_vector<std::pair<int, int>, 7>> m;

[igaztanaga]

Thanks for the report. I've cherry-picked the first commit:

b075bce

Thanks to your test case, I've found another problem: Issue #118. I will add new test cases to test flat_map and flat_set as adaptors.