-The delta is now kept explicitly.
     - This makes delta writes much faster than they used to be, but they use more memory
     - fact-base now exports :delta instead of :last-saved
     - insert! on a fact-base now adds an entry to both :history and :delta
     - still need to do a bit of cleanup around read!
-Rejigged the package.lisp file
-Added a demo usage example to README.md
-`write!` and `write-delta!` now return the name of the file they wrote to
-`multi-insert!` now returns the id of the entry group it inserted