Today, sped up many git-annex commands by around 5%. Often git-annex
traverses the work tree and deserializes keys to its Key data type, only to
turn around and do something with a Key that needs it to be serialized
again. So caching the original serialization of a key avoids that work. I
had started on this in January but had to throw my first attempt away.

The big bytestring conversion in January only yielded a 5-15% speedup,
so an extra 5% is a nice bonus for so relativly little work today.
It also feels like this optimisation approach is nearly paid out though;
only converting all filepath operations to bytestrings seems likely to
yield a similar widespread improvement.
