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. 

Benchmarking `git-annex find`, speedups range from 28-66%. The files fly by
much more snappily. Other commands likely also speed up, but do more work
than find so the improvement is not as large.

The `bs` branch is in a mergeable state now, but still needs work:

* 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.

  As a first step, profile and look for the hot spots. Known hot spots:
  
  * keyFile uses fromRawFilePath and that adds around 3% overhead in `git-annex find`. 
    Converting it to a RawFilePath needs a version of `</>` for RawFilePaths.
  * getJournalFileStale uses fromRawFilePath, and adds 3-5% overhead in
    `git-annex whereis`. Converting it to RawFilePath needs a version 
    of `</>` for RawFilePaths. It also needs a ByteString.readFile
    for RawFilePath.

* 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.)