Replacement for gstat(8)

This is a gstat replacement, written in Rust.  It is more featureful than the
original, and works much better on systems with large numbers of disks.  The
key differences are:

* gstat-rs supports sorting the disks using the '+', '-', and 'r' keys, and the
  "--sort" and "-r" command line options.
* gstat-rs can enable/disable columns at any time using the insert and
  delete keys.  gstat can only do that at startup, and only for certain
  infrequently used columns.
* If the screen has enough space, gstat-rs will display multiple disks side by
  side.
* gstat-rs can pause the display without exiting the program.
* gstat-rs's settings are automatically persisted to a config file.
* gstat-rs does not support batch mode (`-bBC`) output.  If you want that kind
  of information, use iostat(8) instead.
* gstat-rs does not display GEOM consumers (`-c`), but it can easily be
  added if there's any demand for that feature.