Merge decompressor into this repo

Is there any value in keeping two tightly-coupled repositories separate: the `rust-brotli` and [rust-brotli-decompressor](https://github.com/dropbox/rust-brotli-decompressor)? It seems like they should just be two separate crates as sub-dirs in the same repository, using `workplace` cargo feature to manage all connections, and it can still be published separately if needed. This way all changes would go in in sync, and a single git's revision would indicate the state of all brotli code.