added function definiition

after this upgrade, alonzo should be able to handle the following:

    $ ./dist/build/alonzo/alonzo
    Lisp>>> (define (f x y) (+ x y))      // (lambda ("x" "y") ... )
    Lisp>>> (f 1 2)                       // 3
    Lisp>>> (f 1 2 3)                     // Expected 2 args: found values 1 2 3
    Lisp>>> (f 1)                         // Expected 2 args: found values 1
    Lisp>>> (define (factorial x)
        (if (= x 1) 1
            (* x (factorial (- x 1)))))   // (lambda ("x") ... )
    Lisp>>> (factorial 10)                // 3628800
    Lisp>>> (define (counter inc)
        (lambda (x)
            (set! inc (+ x inc)) inc))    // (lambda ("inc") ... )
    Lisp>>> (define my-count
        (counter 5))                      // (lambda ("x") ... )
    Lisp>>> (my-count 3)                  // 8
    Lisp>>> (my-count 6)                  // 14
    Lisp>>> (my-count 5)                  // 19