Compactmap - Vec-based map that uses usize as key type and maintains internal linked list for removed nodes.

You don't choose the key when inserting a new value. You can remove any entry.

Based on [this post](https://play.rust-lang.org/?gist=599f79559d6f18cc0266&version=stable) by [eddyb](https://github.com/eddyb).

The function and structure of CompactMap is almost the same as [Slab](https://docs.rs/slab) apart from missing cached length and more features. If I knew about Slab earlier, CompactMap wouldn't have appeared.

TODO:

* More thorough tests
* Entry (is it really needed?)

License is MIT or Apache, like for Rust itself.