Announce column separator

[moritz-gross]

No description provided.

[moritz-gross]

@NSoiffer some questions:

• how should we announce the separator? Just as "separator" ?
• the way add_builtin_functions works is a bit unclear to me. First off, shouldn't it be called something like register_mathcat_xpath_functions, as we register MathCAT-specific XPath functionality? Also, I think it's a bit clunky that these functions need to be attached to an otherwise empty struct each time. Is there no way around that?
• Isn't the signature cleaner when count_table_dims returns a tuple of usize or i32 etc? idk how I feel about Value<'d> (tbh I need to learn a bit more Rust in the first place to understand what 'd is doing in the first place.