auto merge of #7912 : graydon/rust/extra-new-benchmarks-1, r=catamorphism

This adds new #[bench] benchmarks for extra::smallintmap, treemap, sha1, sha256 and 512, and base64. Also fixes a bunch of warnings in bitv.

diff --cc src/libextra/smallintmap.rs
Simple merge

diff --cc src/libextra/treemap.rs
Simple merge