reverse_fold has no static assert for requiring bidirectional sequence

~~The example from documentation https://www.boost.org/doc/libs/1_69_0/libs/fusion/doc/html/fusion/algorithm/iteration/functions/reverse_fold.html compiles fine as-is, but replacing `vector` with `cons` yield a compile error. This seems to be a regression, because I came to it at debugging failure in Proto tests, but it might be introduced a long time ago.~~

Update: the Proto test/example is simply violating Fusion invariants. I opened a PR to fix it https://github.com/boostorg/proto/pull/15.