Use std::collections::{BTreeSet, BTreeMap} instead of std::collections::{TreeSet, TreeMap}.
Merge branch 'pr-btree' of https://github.com/akiss77/cargo into akiss77-pr-btree
Update to rust master
auto merge of #1079 : alexcrichton/cargo/update-rust, r=alexcrichton

Continuation of #1077
Closes https://github.com/rust-lang/cargo/pull/1079