Change 'let' for better layout

Summary:

(define (g)
  (let ((x E1)
        (y E2))
    (f x y)))

Becomes:

(define (g)
   (let x E1)
   (let y E2)
   (f x y))

Incidentally, I've reindented all of lumgua.lisp to use 3 spaces per level.