Adds variadic arguments in apply_primitive

[vdloo]

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Previous behaviour:

(+ 5 10 15) 15 (- 5 10 15) -5 (* 5 10 15) 50 (/ 100 5 10) 20

New behaviour:

(+ 5 10 15) 30 (- 5 10 15) -20 (* 5 10 15) 750 (/ 100 5 10) 2

[vdloo]

The join procedure from the core library gets trapped in an infinite loop with this change. Any idea what might be causing this? edit: never mind, found it. Next commit fixes it