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:

* There's a bug impacting WORM keys with / in the keyname.
  The files stored in the git-annex branch used to have the `/` changed
  to `_`, but on the bs branch that does not happen. git also outputs
  a message about "Ignoring" the file.

  Test case:

  git config annex.backend WORM
  git annex addurl http://localhost/~joey/index.html

  Hmm, that prints out the Ignoring message, and the file does not get
  written to the git-annex branch. But in my big repo, I saw the message
  and saw a file in the branch, with `/` in its keyname. Earlier in the
  branch, the same key used `_`. (Look for "36bfe385607b32c4d5150404c0" to
  find it again.)

* Profile various commands and look for hot spots.

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