Macro make_udf_function 

macro_rules! make_udf_function {
    ($UDF:ty, $NAME:ident, $CTOR:expr) => { ... };
    ($UDF:ty, $NAME:ident) => { ... };
}

Creates a singleton ScalarUDF of the $UDF function and a function named $NAME which returns that singleton. Optionally use a custom constructor $CTOR which defaults to $UDF::new() if not specified.

This is used to ensure creating the list of ScalarUDF only happens once.