Clean up after the enter-uml transition.
Release v0.9.0.  Changes since the previous release:

* Add the `bedup size-lookup` command to find files by size
* Add the `--defrag` flag for defragmentation (Linux 3.9 recommended)
* Support more than 500 subvolumes by working around a SQLite limitation
* Report clear errors when cli arguments aren't btrfs volumes
* Be more robust against some races when inode numbers are reused
* In `bedup show`, hide deleted volumes by default
* Make sure CFFI modules aren't rebuilt once installed
* Backport syncfs for glibc pre-2.14
* Run CI tests inside UML
* Fix miscellaneous bugs