Unordered: Merge from trunk.

- Some changes to the internals, including reverting some of the
  recent changes to constructing values which turned out to be
  more bother than it was worth.
- On C++11 compilers, better use of `construct` and `destroy`.
- Better testing.


[SVN r80350]

libs/unordered

@@ -1 +1 @@
-Subproject commit 035396e89f0acea5cf2563a203936a5d3a030918
+Subproject commit d5971171da31cdeed7c06bd32e7fe9fc8f86cf51