Fix parts parsing

[Julow]

Fixes #148.

If a part delimiter occur inside a nested structure or signature, it will be ignore:

[@@@part "1"]

module A =
struct

[@@@part "2"]

  type t = A

[@@@part "3"]

end

The parts 2 and 3 won't be parsed and everything will be the body of the part 1, which will be replaced by just:

[@@@part "1"]

module A =
struct

This PR fixes this by parsing the part delimiters line by line using Re instead of using OCaml's parser. This has the advantage of not failing and not depending on an unstable library.

[samoht]

I've restarted the failing CI job.

[samoht]

I think I'm ok with the approach -- it allows to put part in invalid ast fragments too, which is a nice feature (and allow to extract parts of structures as you demonstrated it).

I am not sure to understand where do the extra newlines come from.

Also, what happens if [@@@part ...] doesn't start at the beginning of the line? I got the impression that your patch is able to detect the part but then it removes these spaces in the expected file? I guess this will break ocamlformat-ed files, if you have a part deep into a structure.

[Julow]

The regex that matches [@@@part ...] allow trailing whitespaces but they are not printed back. I didn't change the printing code so this is unchanged, except in the case the file did not parse and the whole file is printed as-is, including part separators.

[Julow]

I added a commit (a21f0fa) to preserve the indentation of [@@@part ...].
Can be reverted. What do you think?

[Julow]

Rebased. @samoht Do you think we can merge this PR ?

[samoht]

LGTM