Define and centralize how comments are interpreted. The new function is
also used by normalization, which had inconsistent rules before.

A few regressions

This fixes two instances of unstable formatting but might be a
regression when `wrap-comments=false`.

This reverts commit 70e5752b06fdef97e3c729d3bbfed14888ece62a.

The indentation of doc comments is significative for verbatim blocks.
The decision of parsing a regular comment as doc must be done before
decoding a comment.

Regressions are due to test cases previously crashing finally being run.

This reverts commit f5cce1a.

No longer the case.

This break was removed in previous commits

Allow indented lines with no asterisks and trailing newline in asterisk
prefixed comments.

A trailing newline results in the star of the closing token to be
aligned with the asterisks.

Don't check the margin to group comments

This could interfere with a comment like:

      (*with exn ->
          failwiths "binary_search bug"
          (exn, `length length, `search_key search_key, `pos pos, `len len)
          <:sexp_of< exn * [ `length of int ] * [ `search_key of int ]
       * [ `pos of int ] * [ `len of int ] >>*)

Preserve comments like:

    (*
    foo
    *)

Comments and docstrings no longer have to be differentiated before
formatting. Concatenating "*" to docstrings is no longer necessary.

Some comments starting with `(**` were in fact not docstrings.

Preliminary to changing its representation.

Comments and docstrings no longer have to be differentiated before
formatting. Concatenating "*" to docstrings is no longer necessary.

Some comments starting with `(**` were in fact not docstrings.
What is a docstring is now dictated by the lexer, which removes this
kind of bug.

Remove the forced line break before a multi-line comment. The asymmetry
of this forced line break allowed comments to be move back and forth
between being attached to after `f` or before `a`.

This adds regressions.

Consecutive comments with no empty line in between are not formatted.