[#34] VistA Functions: handle SQL NULLs and various fixes

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.