Make csv++ aware of all known OpenFormula functions

[patrickomatic]

OpenFormula basically defines the standard library of functions that are common among various spreadsheet implementations: https://docs.oasis-open.org/office/v1.2/os/OpenDocument-v1.2-os-part2.html#__RefHeading__1018044_715980110. Currently csv++ is ignorant of these so it can't inform the user if a function is called with the wrong args or if it does not exist. If I make csv++ aware of these functions (make a crate that scrapes that page and loads them into a BTreeMap) I can make it much much more helpful by being able to warn about:

• calling functions that don't exist
• calling functions with the wrong number of args
• maybe one day, enforce the types of args