Function builtin_union 

fn builtin_union(
    args: &[RValue],
    _: &[(String, RValue)],
) -> Result<RValue, RError>

Set union: unique elements from both x and y.

Works on character, integer, and double vectors. For numeric inputs, returns a numeric vector; for character inputs, returns a character vector.

@param x vector @param y vector @return vector of all unique elements