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.