Use raise_with_backtrace instead of raise

[Willenbrink]

Hey,

when an exception is raised within a continuation the backtrace is lost as we catch the exception and simply re-raise it. This PR exchanges Stdlib.raise by Printexc.raise_with_backtrace so this information is not lost.
I'm not sure exactly how large the performance impact is but it should be insignificant. In addition, I assume it is even cheaper if backtraces are currently not recorded or the exception was raised with Stdlib.raise_notrace.

[favonia]

It seems the backtrace is not lost during the re-raising, at least in newer OCaml. I wonder if you have a concrete example showing the loss? (I made this mistake before...)

[Willenbrink]

I think this is unrelated as I can reproduce the problem on your branch. The problem I fixed is shown by this program (found in test/passing/main.ml on this branch):

open Stdlib.EffectHandlers.Deep

exception%effect E: unit

let () =
  Printexc.record_backtrace true;
  let raiser () = raise Not_found in
  match raiser () with
  | e -> e
  | [%effect? E, k] ->
    continue k ()

Without my patch this prints:

Fatal error: exception Not_found    
Raised at Ppx_effects_runtime.raise in file "src/ppx_effects_runtime.ml", line 4, characters 12-24
Called from Dune__exe__Main in file "test/passing/main.ml", line 8, characters 2-74

With the patch it results in:

Fatal error: exception Not_found    
Raised at Dune__exe__Main.raiser in file "test/passing/main.ml", line 7, characters 18-33
Re-raised at Ppx_effects_runtime.raise in file "src/ppx_effects_runtime.ml", line 6, characters 2-36
Called from Dune__exe__Main in file "test/passing/main.ml", line 8, characters 2-74

As you can see the original raise is not present in the first backtrace but is in the second one. I did notice that my patch only fixes this for match with but not for try with. Looking at the code I can not find an obvious fix for this although it is somehow related to this block as we never call exnc with the correct raise there.