In preparation for landing `string` support in `wiggle`, I've
revisited `GuestArray` and the deref mechanics, and decided to
scrap the current approach in favour of the previous. Namely, for
`T: GuestTypeCopy` we provide `GuestArray::as_ref` which then can
be derefed directly to `&[T]` since the guest and host types have
matching representation, and for other types (thinking here of
our infamous arrays of pointers), I've instead added a way to
iterate over the pointers. Then, it is up to the `wiggle`'s client
to know whether they can deref whatever the pointer stored in array
is pointing at.
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