more compact formatting for while-loops with no body

[v-gb]

Concretely, this is what changes:

 while
   loooong
     condition
-do
-  ()
-done
+do () done

A real example where I wished for a more compact formatting:

ocaml/ocaml-re@59198dd#diff-a91a869b078e8995843c8b377da588505468c962166d62d7029893c27be32833R70-R80

To explain a bit: while-loops are fairly inexpressive in ocaml due to the absence of break. In a decent number of cases, it's nicer to give up on the separation between condition and body, by moving the body into the condition. In that case, it's a bit silly to waste 3 lines on the empty body.

For instance, if you wanted to write a loop where the first iteration always runs, like a do-while in C, the obvious thing to do (without auxiliary functions) is:

let first = ref true in
while !first || condition; do first := false; body done

but the simpler version is:

while body; condition do () done

Or iterating through a stack:

while not (Stack.is_empty s) do f (Stack.pop_exn s) done

can be done without double-checking that the stack is empty:

while
  match Stack.pop s with
  | None -> false
  | Some elt -> f elt; true
do () done

[EmileTrotignon]

Not sure why the CI is failing, but I agree with the goal. The tests and implementation are good.

[v-gb]

Thanks! Right, the failures are unrelated to the change, though no idea either why they are occurring.

I added the PR number to the CHANGES file (it's going to incidentally retrigger builds, but I expect they will fail again).

[Julow]

Looks good! The CI should run if you merge the main branch.

[v-gb]

After rebase, there's still some kind of unrelated failure.

[Julow]

Thanks! The remaining failures are not related, I'll fix them separately.