Another fun project to work on.  Experiment with the use of lazy
bytestrings in darcs, especially now that we are using bytestrings
already.

Also, as Petr Ročkai noticed on IRC:
  mornfall: mmapFileByteStringLazy oh wow.
  mornfall: Maps region of file and returns it as
    Data.ByteString.Lazy.ByteString. File is mapped in in ReadOnly mode. See
  mmapFilePtrLazy for details. Chunks are mapped in on demand.
  mornfall: And it works on windows.
  mornfall: And the lazy zlib binding. Mmm.

David's notes on it (I think):

Ideally, we'd do this after introducing the new hunk patches. Also note that I
do not want lazy IO, as that's been a major source of headaches in terms of
problems like file handle leaks. But O(1) append would be nice, as would the
highly-optimized nature of bytestring.

Making Don nosy (maybe of interest?)

I've added issue1357 to our superseder list because of David's suggestion that
it would be ideal to get chunky hunks first, but I don't really understand why
he thinks this is the case, maybe that the benefits would only really kick in
once we have them?

I'd recommend that we only do this with extensive benchmarking and also some
fairly detailed argumentation about when we should/shouldn't be using these lazy
bytestrings.