no code changes

And add a simple dots-based progress display, currently only used in v2
upgrade.

Did all sources except Remotes/* and Command/*

The only remaining vestiage of backends is different types of keys. These
are still called "backends", mostly to avoid needing to change user interface
and configuration. But everything to do with storing keys in different
backends was gone; instead different types of remotes are used.

In the refactoring, lots of code was moved out of odd corners like
Backend.File, to closer to where it's used, like Command.Drop and
Command.Fsck. Quite a lot of dead code was removed. Several data structures
became simpler, which may result in better runtime efficiency. There should
be no user-visible changes.

Add --fast flag, that can enable less expensive, but also less thurough versions of some commands.

* Add --fast flag, that can enable less expensive, but also less thurough
  versions of some commands.
* fsck: In fast mode, avoid checking checksums.
* unused: In fast mode, just show all existing temp files as unused,
  and avoid expensive scan for other unused content.

It compiles. It sorta works. Several subcommands are FIXME marked and
broken, because things that used to accept separate --backend and --key
params need to be changed to accept just a --key that encodes all the key
info, now that there is metadata in keys.

Rethink filename encoding handling for display. Since filename encoding may or may not match locale settings, any attempt to decode filenames  will fail for some files. So instead, do all output in binary mode.