Changed to a binary index file, written using Storable, for speed

Changed to a binary index file, written using Storable, for speed

During refresh of a wiki with 800 files, loadindex was using more total
time than any other function, and saveindex was also in the top ten.
Rewriting them to use Storable makes them three times as fast.

0.7 seconds is saved on my laptop in profiling mode.