Fix lambdas in head position

This fixes the first and last examples below, both of which produced an
error before:

> ((lambda (x) x) 2)
2
> (define comp
..  (lambda (f g)
..    (lambda (x)
..      (f (g x)))))
nil
> (define double (lambda (x) (+ x x)))
nil
> ((comp double double) 2)
8