[Tree unique] Simplify loops to not have inputs

[oflatt]

It turns out that loop inputs are unnecessary, and we can just use Let. So this rule removes inputs from loops. For the first iteration, the argument in the loop refer to the argument outside the loop.

For example, the following loop:

(Loop (Id 1)
  (Add (Num 1) (Num 2))
   some_loop_body)

Can be expressed as

(Let (Id 2)
  (Add (Num 1) (Num 2))
    (Loop (Id 1) some_loop_body))

In general, the following rule holds:

(rule ((Loop inputs outputs))
      ((let new-id (i64-fresh!))
       (union (Loop inputs outputs)
              (NewLet new-id inputs (Loop (Arg new-id) outputs)))))

[oflatt]

Converting this to a draft- I think we should consider it in the future but not implement it right now.

[oflatt]

Closing, I think it's more natural for loops to have inputs like other operators