Expose utils for DiskStats

[adinapoli]

Fixes #53. We can now grab the disk stats:

> stats <- getDiskStats (\_ _ -> pure ())
> stats
[
    DiskStats {
      _diskFilesystem = "/dev/disk1s1s1"
    , _diskUsedBytes = 3.0958456e7
    , _diskUsedPercent = 6
    , _diskFreeBytes = 4.91945976e8
    , _diskFreePercent = 94
  }
  ,DiskStats {
      _diskFilesystem = "/dev/disk1s3"
    , _diskUsedBytes = 9999192.0
    , _diskUsedPercent = 2
    , _diskFreeBytes = 4.91945976e8
    , _diskFreePercent = 98
  }
  ,DiskStats {
      _diskFilesystem = "/dev/disk1s5"
    , _diskUsedBytes = 48.0
    , _diskUsedPercent = 0
    , _diskFreeBytes = 4.91945976e8
    , _diskFreePercent = 100
  }
  ,DiskStats {
      _diskFilesystem = "/dev/disk1s6"
    , _diskUsedBytes = 139736.0
    , _diskUsedPercent = 0
    , _diskFreeBytes = 4.91945976e8
    , _diskFreePercent = 100
  }
  ,DiskStats {
      _diskFilesystem = "/dev/disk1s2"
    , _diskUsedBytes = 1.363316528e9
    , _diskUsedPercent = 73
    , _diskFreeBytes = 4.91945976e8
    , _diskFreePercent = 27
  }
  ,DiskStats {
      _diskFilesystem = "/dev/disk1s7"
    , _diskUsedBytes = 5.4268752e7
    , _diskUsedPercent = 10
    , _diskFreeBytes = 4.91945976e8
    , _diskFreePercent = 90
  }
  ,DiskStats {
      _diskFilesystem = "/dev/disk4s2"
    , _diskUsedBytes = 5.28663472e8
    , _diskUsedPercent = 85
    , _diskFreeBytes = 9.5807152e7
    , _diskFreePercent = 15
  }
  ,DiskStats {
      _diskFilesystem = "/dev/disk1s1"
    , _diskUsedBytes = 3.0958456e7
    , _diskUsedPercent = 6
    , _diskFreeBytes = 4.91945976e8
    , _diskFreePercent = 94
  }
]