Add extract-uuid-from-string, quote-plaintext, flatten

Also export their symbols.

'quote-plaintext is being moved in here from sbcl-4store, because it
belongs here more than there, and it's reasonable for sbcl-4store to
expect calling functions to supply correctly-quoted values.
Split ox.asd into ox.asd and ox-tests.asd

Also add a dependency on :split-sequence to ox.asd.
Update tags.lisp and its tests to mostly work

Adding, deleting and checking for tags now work.
Still ignoring the functions for tagging other items, for the moment.