This commit is an attempt at improving the safety of using the return
value of the `SharedMemory::data` method. Previously this returned
`*mut [u8]` which, while correct, is unwieldy and unsafe to work with.
The new return value of `&[UnsafeCell<u8>]` has a few advantages:
* The lifetime of the returned data is now connected to the
`SharedMemory` itself, removing the possibility for a class of errors
of accidentally using the prior `*mut [u8]` beyond its original lifetime.
* It's not possibly to safely access `.len()` as opposed to requiring an
`unsafe` dereference before.
* The data internally within the slice is now what retains the `unsafe`
bits, namely indicating that accessing any memory inside of the
contents returned is `unsafe` but addressing it is safe.
I was inspired by the `wiggle`-based discussion on #5229 and felt it
appropriate to apply a similar change here.
2be457c295