The will make it possible to put a VsCoq toplevel in `ide/vscoq`.

Add headers to a few files which were missing them.

We make the primitives for backtrace-enriched exceptions canonical in
the `Exninfo` module, deprecating all other aliases.

At some point dependencies between `CErrors` and `Exninfo` were a bit
complex, after recent clean-ups the roles seem much clearer so we can
have a single place for `iraise` and `capture`.

The standard use is to repeat the option keywords in lowercase, which
is basically useless.

En passant add doc entry for Dump Arith.

We also remove trailing whitespace.

Script used:

```bash
for i in `find . -name '*.ml' -or -name '*.mli' -or -name '*.mlg'`; do expand -i "$i" | sponge "$i"; sed -e's/[[:space:]]*$//' -i.bak "$i"; done
```

The changes are large due to `Pervasives` deprecation:

- the `Pervasives` module has been deprecated in favor of `Stdlib`, we
  have opted for introducing a few wrapping functions in `Util` and
  just unqualified the rest of occurrences. We avoid the shims as in
  the previous attempt.

- a bug regarding partial application have been fixed.

- some formatting functions have been deprecated, but previous
  versions don't include a replacement, thus the warning has been
  disabled.

We may want to clean up things a bit more, in particular
w.r.t. modules once we can move to OCaml 4.07 as the minimum required
version.

Note that there is a clash between 4.08.0 modules `Option` and `Int`
and Coq's ones. It is not clear if we should resolve that clash or
not, see PR #10469 for more discussion.

On the good side, OCaml 4.08.0 does provide a few interesting
functionalities, including nice new warnings useful for devs.

This is a pre-requisite to use automated formatting tools such as
`ocamlformat`, also, there were quite a few places where the comments
had basically no effect, thus it was confusing for the developer.

p.s: Reading some comments was a lot of fun :)

Even if `fake_ide` was under tools, it depended on libraries from
`ide`. Thus, we move `fake_ide` to `ide`, and make it "private" to the
test-suite [this means `test-suite` depends on the `ide` folder then].

In the Dune side, we reorganize libraries so `fake_ide` doesn't depend
on GTK anymore, this allows to run the test-suite when GTK is not
available.

In order to achieve this, we had to split the `coqide` package in a
server and client version.