Add Polymorphic Memory Resource utilities and rework the separately compiled library:

- Dlmalloc's based C function are boost_cont_xxx no longer exported, but wrapped into C++ linkage dlmalloc_xxx functions to effectively reuse Boost's dynamic library and autolink machinery instead of rewriting machinery to compile the C source file.
- Refactored scoped_allocator_adaptor's construct logic as it was shared with polymorphic allocator's one. Moved common logic to detail/dispatch_uses_allocator.hpp. Refactored also scoped_allocator_adaptor test utilities to be reused with polymorphic_allocator tests.

bench/bench_alloc_stable_vector_burst.cpp

@@ -174,8 +174,8 @@ void stable_vector_test_template(unsigned int num_iterations, unsigned int num_e
                      << " (" << (float(top_capacity)/float(num_iterations*num_elements) - 1)*100 << " %)"*/
                      << std::endl << std::endl;
    }
-   assert(boost_cont_all_deallocated());
-   boost_cont_trim(0);
+   assert(bc::dlmalloc_all_deallocated());
+   bc::dlmalloc_trim(0);
 }
 
 void print_header()