Changes in this release:

* Fix possible double-free in `insert_many` when passed an iterator that
  panics in `next` (#103)
* Add a new `union` feature (disabled by default) that reduces the size
  of the SmallVec struct (#94)
* Improve performance of `extend` and `from_elem` (#93)
* Improve performance of `drop` (#100)
* Update dev-dependency on `bincode` (#102)
* Update to build without `std` on current Rust nightly (#104)
* Additional benchmarks (#95, #97).