Add check for merging let forms

[Fuco1]

If we have nested let forms which can be merged, do so

(let ((a 1))
  (let ((b 2))
    (body)
    (here)))

;; =>

(let ((a 1)
      (b 2))
  (body)
  (here))

We need a little bit more attention with dependencies

(let ((a 1))
  (let ((b a))
    (body)
    (here)))

;; =>

(let* ((a 1)
       (b a))
  (body)
  (here))