Add `: Box<_>` or `::Box<_>` type annotations to various places.

This is the kind of change that one is expected to need to make to
accommodate overloaded-`box`.

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Note that this is not *all* of the changes necessary to accommodate
Issue 22181.  It is merely the subset of those cases where there was
already a let-binding in place that made it easy to add the necesasry
type ascription.

(For unnamed intermediate `Box` values, one must go down a different
route; `Box::new` is the option that maximizes portability, but has
potential inefficiency depending on whether the call is inlined.)

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There is one place worth note, `run-pass/coerce-match.rs`, where I
used an ugly form of `Box<_>` type ascription where I would have
preferred to use `Box::new` to accommodate overloaded-`box`.  I
deliberately did not use `Box::new` here, because that is already done
in coerce-match-calls.rs.

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Precursor for overloaded-`box` and placement-`in`; see Issue 22181.

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