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]

test/unordered/rehash_tests.cpp

@@ -19,7 +19,8 @@ test::seed_t seed(2974);
 template <class X>
 bool postcondition(X const& x, BOOST_DEDUCED_TYPENAME X::size_type n)
 {
-    return x.bucket_count() > x.size() / x.max_load_factor() &&
+    return static_cast<double>(x.bucket_count()) >
+        static_cast<double>(x.size()) / x.max_load_factor() &&
         x.bucket_count() >= n;
 }