Unordered: Reorganization to use void pointers and other things.

Helps allocators which can't use incomplete pointers, and avoid using
base pointers where that might not be possible.  And some other
reorganization. Storing arguments to emplace in a structure when
variadic template parameters aren't available. Changed some of the odd
design for working with older compilers.

[SVN r74742]

test/unordered/constructor_tests.cpp

@@ -15,8 +15,6 @@
 #include "../helpers/input_iterator.hpp"
 #include "../helpers/invariants.hpp"
 
-#include <iostream>
-
 namespace constructor_tests {
 
 test::seed_t seed(356730);