handle a special case of variable shadowing

[tiye]

for a case like:

; "a special case variable shadowing of `b`"
let
    b -1
    a $ loop
        xs $ []
        b 0
      if (>= b 5) xs
        recur (conj xs b) (inc b)
  assert= a $ [] 0 1 2 3 4
  assert= b -1

where b is defined both outside and inside, which reused the return_label of loop recursion. but actually it does not forward value, it only returns. the outer context should be unaware for code in bind function.