Non-idempotency for comment being moved to same line about verusfmt

[parno]

The problem stems from trying to distinguish different non-line single-line comments. For example, in this program:

// Comment
const x: int = 5;

impl a {
    // My favorite function
    fn b()
    //
    ;
}

None of the comments are considered inline, since they all appear on their own dedicated line. The logic in comment_to_doc thus formats the comment as is, without doing any of the special fixups we do for inline comments, on the assumption that the outer context will handle the appropriate newlines. This works well for the first two cases where the comments are "top level" in a sense, but in the final comment, we're in the midst of the function signature, so we don't add a newline. Then the second run of verusfmt notices what is now an inline comment and adjusts it to be spaced appropriately.

I just pushed 5395ef2, which fixes this particular example, but it may create issues for other creative places people decide to add comments in the midst of function declarations.