IMPORTANT: I think this PR fixes an error in lambda_to_flambda, but I am not sure about that and it needs a careful review.

Currently when generating an apply_cont in lambda_to_flambda, we first compute the extra args before calling restore_continuation_context, which can change the continuation being called and thus the extra args that are needed. It seems to me it should be the other way around, but there could be subtleties I'm missing.

Unfortunately, the only way I know to trigger the bug is to use a WIP version of unboxed products, in which the strange treatment we currently have of those (passing them to every continuation like mutable variables) has not been undone, which is available here: Ekdohibs@0abdaa8 .

With that commit and -extension layouts_beta options, this program (found by @ccasin) crashes flambda due to continuations with incorrect number of arguments, while it works with this commit on top:

let rec f6_2 n (tup : #(int * int)) =
  if (Sys.opaque_identity fst) n = 0 then tup
  else tup

I tried to produce a similar version with mutable variables, but have not succeeded yet.