Change the return type of SharedMemory::data (#5240)

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.

crates/fuzzing/src/oracles/diff_wasmtime.rs

@@ -169,12 +169,11 @@ impl DiffInstance for WasmtimeInstance {
 
     fn get_memory(&mut self, name: &str, shared: bool) -> Option<Vec<u8>> {
         Some(if shared {
-            let data = self
+            let memory = self
                 .instance
                 .get_shared_memory(&mut self.store, name)
-                .unwrap()
-                .data();
-            unsafe { (*data).to_vec() }
+                .unwrap();
+            memory.data().iter().map(|i| unsafe { *i.get() }).collect()
         } else {
             self.instance
                 .get_memory(&mut self.store, name)