Tidying in advance of next change
Move all background-validation stuff into the parsing directories
Remove dead code.
* The form of a background-changer within a fact is now more flexible.
* Background can take a let-style list
Add doc for "background changers" + 2 new guides
Better error message
Watch out for left-hand sides that are lazyseqs instead of lists.
Little utility that allows either a singleton vector or sequence of args
Get rid of annoying output from `unfinished`
Get rid of `when-valid` in favor of `assert-right-shape!`
Little helper function
Comment out old background tests; will build them up again
Formula test commented out for now
Tidiness
The "right shape" for background forms has been made more forgiving
`background` and `against-background` do error checking the new way
Comment out validation code that needs migration