Merge unordered from trunk.

- Avoid using operator& with the value type.
- More comments in headers.
- Remove old clang workaround.
- Adjust use of inline to make Borland a little happier.
- Avoid `-Wconversion` warnings.


[SVN r67663]

libs/unordered

@@ -1 +1 @@
-Subproject commit dc8e65043b49136c5ebf3b54d54d211974a59cf8
+Subproject commit 54f9626c1298d46e28d89f5ba7aca3155a2f62b7