add syntatic-sugar for multi-arg λs

(λ (a b) (+ a b))  =>  (λ a (λ b (+ a b))
(λ a (+ a b))      =>  (λ a (+ a b))       # this syntax is kept
(λ a (+ a b))     <=>  (λ (a) (+ a b))     # these two are equivalent
make-int function