rewriter: support rewriting pointers to types like Box<T>

[aneksteind]

At some point we'll need to modify the rewriting of hypothetical lifetimes to handle cases where a pointer in the original struct becomes something other than a reference (such as Box<T>, or in this case being left as *mut T), but this should work well enough for now.

Originally posted by @spernsteiner in #960 (comment)