Drop variadic functions

[michaelhkay]

We've made a decision -- for good reasons of maintaining extensibility -- not to make any of the functions in the system function library variadic.

I think this raises the question of whether the feature is worth retaining. And we already had the situation that many functions that were obvious candidates for variadicity (min, max, distinct-values, sum) couldn't use the feature because they already had additional optional arguments.

If it's not a good design principle for system-defined functions, then the same applies equally to user-defined functions.

We could drop the feature and just revert to defining concat() as a special case. It would remove a significant amount of complexity.