This removes most explicit uses of the + argument mode. Pending a
snapshot, I had to remove the forbid(deprecated_modes) pragma from
a bunch of files. I'll put it back!
+ mode still has to be used in a few places for functions that get
moved (see task.rs)
The changes outside core and std are due to the to_bytes trait and
making the compiler (with legacy modes on) agree with the libraries
(with legacy modes off) about modes.
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.
