Keep pace with some Rust changes

This doesn't actually include the Rust version update (to 6791021) because the
feature for which I wanted it (debug info) doesn't work, and there's no point
making everyone rebuild until it does.
auto merge of #755 : kmcallister/servo/zip_copies, r=metajack

This doesn't actually include the Rust version update (to `6791021`) because the feature for which I wanted it (debug info) doesn't work, and there's no point making everyone rebuild until it does.