The Mut<T> type is intended to allow freezable data stuctures to be stored in
`@mut` boxes. Currently this causes borrowck to be very conserivative since it
cannot prove that you are not modifying such a structure while iterating over
it, for example. But if you do `@Mut<T>` instead of `@mut T`, you will
effectively convert borrowck's static checks into dynamic ones. This lets
you use the e.g. send_map just like a Java Map or something else.
