git-annex uses FilePath (String) extensively. That's a slow data type.
Converting to ByteString, and RawFilePath, should speed it up
significantly, according to [[/profiling]].

I've made a test branch, `bs`, to see what kind of performance improvement
to expect. Most commands don't built yet in that branch, but `git annex
find` does. Speedups range from 28-66%. The files fly by much more
snappily.

As well as adding back all the code that was disabled to get it to build,
the `bs` branch has quite a lot of things still needing work, including:

* Eliminate all the fromRawFilePath, toRawFilePath, encodeBS,
  decodeBS conversions. Or at least most of them. There are likely
  quite a few places where a value is converted back and forth several times.

  It would be good to instrument them with Debug.Trace and find out which
  are the hot ones that get called, and focus on those.

* System.FilePath is not available for RawFilePath, and many of the
  conversions are to get a FilePath in order to use that library.

  It should be entirely straightforward to make a version of System.FilePath
  that can operate on RawFilePath, except possibly there could be some
  complications due to Windows.

* Use versions of IO actions like getFileStatus that take a RawFilePath,
  avoiding a conversion. Note that these are only available on unix, not
  windows, so a compatability shim will be needed.
  (I can't seem to find any library that provides one.)

* Eliminate some Data.ByteString.Lazy.toStrict, which is a slow copy.

* Use ByteString for parsing git config to speed up startup.