[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
...
```

1 files changed, 2 insertions, 2 deletions

diff --git a/vernac/topfmt.ml b/vernac/topfmt.ml
index 45f40b1258..de02f7ecfb 100644
--- a/vernac/topfmt.ml
+++ b/vernac/topfmt.ml
@@ -359,7 +359,7 @@ let in_phase ~phase f x =
     default_phase := op;
     res
   with exn ->
-    let iexn = Backtrace.add_backtrace exn in
+    let iexn = Exninfo.capture exn in
     default_phase := op;
     Util.iraise iexn
 
@@ -415,7 +415,7 @@ let with_output_to_file fname func input =
     close_out channel;
     output
   with reraise ->
-    let reraise = Backtrace.add_backtrace reraise in
+    let reraise = Exninfo.capture reraise in
     std_ft := Util.pi1 old_fmt;
     err_ft := Util.pi2 old_fmt;
     deep_ft := Util.pi3 old_fmt;