Refine the presentation of warnings

[Yoric]

I see:

File "src/foo/bar.ml", lines 39-51, characters 0-34
Warning (GITLAB+1): bla, bla

val foo: ...
      -> ...
      -> ...
      -> ...
^^^^^^^

I'd like to see:

val foo: ...
      -> ...
      -> ...
      -> ...
^^^^^^^

File "src/foo/bar.ml", lines 39-51, characters 0-34
Warning (gitlab-#phantom_type_var): bla, bla

(disable this warning with ...)

• The arrows are pointing from the warning to the offending site, rather than from the next warning/error message to the offending site.
• The SCREAMING CASE is a bit hard on the eye.
• The number is not particularly useful.
• Instructions on disabling the warning are useful, though.