Document and test no_std support This builds on the new "std" Cargo feature added in #49. <!-- Reviewable:start --> --- This change is [<img src="https://reviewable.io/review_button.svg" height="34" align="absmiddle" alt="Reviewable"/>](https://reviewable.io/reviews/servo/rust-smallvec/53) <!-- Reviewable:end -->
Documentation
"Small vector" optimization for Rust: store up to a small number of items on the stack