[ocaml] Remove Custom Backtrace module in favor of OCaml's

As suggested by Pierre-Marie Pédrot, this is a more conservative
version of #8771 .

In this commit, we replace Coq's custom backtrace type with OCaml
`Printexc.raw_backtrace`; this seems to already give some improvements
in terms of backtraces [see below] and removes quite a bit of code.

Main difference in terms of API is that backtraces become now
first-class in `Exninfo`, and we seek to consolidate thus the
exception-related APIs in that module.

We also fix a bug in `vernac.ml` where the backtrace captured was the
one of `edit_at`.

Closes #6446

Example with backtrace from https://github.com/coq/coq/issues/11366

Old:

```
raise @ file "stdlib.ml", line 33, characters 17-33
frame @ file "pretyping/coercion.ml", line 406, characters 24-68
frame @ file "list.ml", line 117, characters 24-34
frame @ file "pretyping/coercion.ml", line 393, characters 4-1004
frame @ file "pretyping/coercion.ml", line 450, characters 12-40
raise @ unknown
frame @ file "pretyping/coercion.ml", line 464, characters 6-46
raise @ unknown
frame @ file "pretyping/pretyping.ml", line 839, characters 33-95
frame @ file "pretyping/pretyping.ml", line 875, characters 50-94
frame @ file "pretyping/pretyping.ml", line 1280, characters 2-81
frame @ file "pretyping/pretyping.ml", line 1342, characters 20-71
frame @ file "vernac/vernacentries.ml", line 1579, characters 17-48
frame @ file "vernac/vernacentries.ml", line 2215, characters 8-49
frame @ file "vernac/vernacinterp.ml", line 45, characters 4-13
...
```

New:

```
Raised at file "stdlib.ml", line 33, characters 17-33
Called from file "pretyping/coercion.ml", line 406, characters 24-68
Called from file "list.ml", line 117, characters 24-34
Called from file "pretyping/coercion.ml", line 393, characters 4-1004
Called from file "pretyping/coercion.ml", line 450, characters 12-40
Called from file "pretyping/coercion.ml", line 464, characters 6-46
Called from file "pretyping/pretyping.ml", line 839, characters 33-95
Called from file "pretyping/pretyping.ml", line 875, characters 50-94
Called from file "pretyping/pretyping.ml" (inlined), line 1280, characters 2-81
Called from file "pretyping/pretyping.ml", line 1294, characters 21-94
Called from file "pretyping/pretyping.ml", line 1342, characters 20-71
Called from file "vernac/vernacentries.ml", line 1579, characters 17-48
Called from file "vernac/vernacentries.ml", line 2215, characters 8-49
Called from file "vernac/vernacinterp.ml", line 45, characters 4-13
...
```

4 files changed, 9 insertions, 11 deletions

diff --git a/lib/cErrors.ml b/lib/cErrors.ml
index b9735d0579..9f496f5845 100644
--- a/lib/cErrors.ml
+++ b/lib/cErrors.ml
@@ -12,7 +12,7 @@ open Pp
 
 (** Aliases *)
 
-let push = Backtrace.add_backtrace
+let push = Exninfo.capture
 
 (* Errors *)
 
@@ -51,12 +51,10 @@ let raw_anomaly e = match e with
   | _ ->
     str "Uncaught exception " ++ str (Printexc.to_string e) ++ str "."
 
-let print_backtrace e = match Backtrace.get_backtrace e with
+let print_backtrace e = match Exninfo.get_backtrace e with
 | None -> mt ()
 | Some bt ->
-  let bt = Backtrace.repr bt in
-  let pr_frame f = str (Backtrace.print_frame f) in
-  let bt = prlist_with_sep fnl pr_frame bt in
+  let bt = str (Exninfo.backtrace_to_string bt) in
   fnl () ++ hov 0 bt
 
 let print_anomaly askreport e =
diff --git a/lib/control.ml b/lib/control.ml
index 7d54838df8..e67e88ee95 100644
--- a/lib/control.ml
+++ b/lib/control.ml
@@ -38,7 +38,7 @@ let unix_timeout n f x e =
     restore_timeout ();
     res
   with e ->
-    let e = Backtrace.add_backtrace e in
+    let e = Exninfo.capture e in
     restore_timeout ();
     Exninfo.iraise e
 
@@ -76,7 +76,7 @@ let windows_timeout n f x e =
     else raise e
   | e ->
     let () = killed := true in
-    let e = Backtrace.add_backtrace e in
+    let e = Exninfo.capture e in
     Exninfo.iraise e
 
 type timeout = { timeout : 'a 'b. int -> ('a -> 'b) -> 'a -> exn -> 'b }
@@ -102,7 +102,7 @@ let protect_sigalrm f x =
       | true, Sys.Signal_handle f -> f Sys.sigalrm; res
       | _, _ -> res
     with e ->
-      let e = Backtrace.add_backtrace e in
+      let e = Exninfo.capture e in
       Sys.set_signal Sys.sigalrm old_handler;
       Exninfo.iraise e
   with Invalid_argument _ -> (* This happens on Windows, as handling SIGALRM does not seem supported *)
diff --git a/lib/flags.ml b/lib/flags.ml
index b87ba46634..ad48024761 100644
--- a/lib/flags.ml
+++ b/lib/flags.ml
@@ -19,7 +19,7 @@ let with_modified_ref ?(restore=true) r nf f x =
     if restore || pre == !r then r := old_ref;
     res
   with reraise ->
-    let reraise = Backtrace.add_backtrace reraise in
+    let reraise = Exninfo.capture reraise in
     r := old_ref;
     Exninfo.iraise reraise
 
@@ -37,7 +37,7 @@ let with_options ol f x =
     let r = f x in
     let () = List.iter2 (:=) ol vl in r
   with reraise ->
-    let reraise = Backtrace.add_backtrace reraise in
+    let reraise = Exninfo.capture reraise in
     let () = List.iter2 (:=) ol vl in
     Exninfo.iraise reraise
 
diff --git a/lib/pp.ml b/lib/pp.ml
index 3e9ab2a82b..1bd160dcda 100644
--- a/lib/pp.ml
+++ b/lib/pp.ml
@@ -203,7 +203,7 @@ let pp_with ft pp =
   in
   try pp_cmd pp
   with reraise ->
-    let reraise = Backtrace.add_backtrace reraise in
+    let reraise = Exninfo.capture reraise in
     let () = Format.pp_print_flush ft () in
     Exninfo.iraise reraise