Allow reflection of Box<[T]>

[nrc]

Box<[T]> falls out from DST so should work, and mostly it does. Unfortunately, the code for reflecting it got torn out before DST landed and I don't want to delay landing DST any longer for an unpopular feature on an unpopular type. This should be easy to fix.

OTOH, reflection of unsized values doesn't work due to an implementation choice in reflection, so making Smaht<[T]> will be harder to fix, as would doing Box<[T]> properly.

All this probably applies equally to Box<str> too.

[nrc]

We're removing reflection (#18064), so this no longer applies.