Upgrade to some components of C++11.

The first changeset removes `Moveable<T>` for `&&`. The second removes `NULL` for `nullptr`. This does not change much for AM (yet) since the minimum compiler versions are still pretty low (GCC >= 4.3, MSVC >= 2010, Clang >= 3.0), though downstream projects will have to fix any code that uses `Moveable`.