Macro export_functions

macro_rules! export_functions {
    ($(($FUNC:ident, $DOC:expr, $($arg:tt)*)),*) => { ... };
    (single $FUNC:ident, $DOC:expr, $arg:ident,) => { ... };
    (single $FUNC:ident, $DOC:expr, $($arg:ident)*) => { ... };
}

macro that exports a list of function names as:

1. individual functions in an expr_fn module
2. a single function that returns a list of all functions

Equivalent to

pub mod expr_fn {
    use super::*;
    /// Return encode(arg)
    pub fn encode(args: Vec<Expr>) -> Expr {
        super::encode().call(args)
    }
 ...
/// Return a list of all functions in this package
pub(crate) fn functions() -> Vec<Arc<ScalarUDF>> {
    vec![
      encode(),
      decode()
   ]
}

Exported functions accept:

• Vec<Expr> argument (single argument followed by a comma)
• Variable number of Expr arguments (zero or more arguments, must be without commas)