This MR does several things:

- All functions that can take a parameter that can be an SQL NULL will
  handle it appropriately; mostly that is just returning SQL NULL back,
  but in `IFNULL` doing the appropriate substitution.
- A forthcoming Octo version will contain SUBSTRING; so it's now removed
  from here.
- The functions sql defintion file would only work on a much earlier
  version of Octo that did not enforce the function types. As of commit
  18bce25f541c0d0c0087bb77d691db9b3b25dd15 (Aug 2020), this file
  wouldn't have worked anymore, as that commit made functions fully
  typed. The file is now updated so that all functions are typed
  properly.
- `RIGHTY` functions behaved like `LEFTY`; that's now fixed; and code
  has been simplified so that both functions are essentially
  passthroughs to $EXTRACT.
- `REPLACE` simplified to use pre-existing VistA code.