[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
...
```

0 files changed, 0 insertions, 0 deletions