[pp] Ensure Format is called with top level boxes.

The documentation of Ocaml's `Format` module specifies:

> The behaviour of pretty-printing commands is unspecified if there is
> no opened pretty-printing box. Each box opened via one of the open_
> functions below must be closed using close_box for proper
> formatting. Otherwise, some of the material printed in the boxes may
> not be output, or may be formatted incorrectly.

(http://caml.inria.fr/pub/docs/manual-ocaml/libref/Format.html)

However, [lib/pp.ml] seems not to take lots of precautions to ensure
that format is always called with an enclosing box.

If `log_via_feedback` is set, [lib/pp.ml:string_of_ppcmds] will call
`msg_with` without a top-level enclosing box, resulting in poor display
results like mismatched width.

Note that there are more functions affected, but these codepaths don't
seem to matter when `log_via_feedback` is set.

A small cleanup of pp.ml may be in order.

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