Make handling of `Comments` more iterator-like.

The current way of stepping through each comment in `Comments` is a bit
weird. There is a `Vec<Comments>` and a `current` index, which is fine.
The `Comments::next` method clones the current comment but doesn't
advance `current`; the advancing instead happens in `print_comment`,
which is where each cloned comment is actually finally used (or not, in
some cases, if the comment fails to satisfy a predicate).

This commit makes things more iterator-like:
- `Comments::next` now advances `current` instead of `print_comment`.
- `Comments::peek` is added so you can inspect a comment and check a
  predicate without consuming it.
- This requires splitting `PrintState::comments` into immutable and
  mutable versions. The commit also moves the ref inside the `Option` of
  the return type, to save callers from having to use `as_ref`/`as_mut`.
- It also requires adding `PrintState::peek_comment` alongside the
  existing `PrintState::next_comment`. (The lifetimes in the signature
  of `peek_comment` ended up more complex than I expected.)

We now have a neat separation between consuming (`next`) and
non-consuming (`peek`) uses of each comment. As well as being clearer,
this will facilitate the next commit that avoids unnecessary cloning.

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