[vm/ffi] Deprecate Struct.addressOf

Now that we have nested structs, objects a subtype of `Struct` can be
backed by either a `Pointer` or a `TypedData`. Having this accessor is
misleading.

Instead of passing a struct around (which could be backed by either),
the `Pointer<T extends Struct>` should be passed around and `.ref`
should be used everywhere when access to the backing pointer is
required.

Issue: #40667

Related issues:
* Optimize .ref #38648
* Support tree shaking of structs #38721

Change-Id: I3c73423480b91c00639df886bf1d6ac2e444beab
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/177581
Reviewed-by: Martin Kustermann <[email protected]>
Commit-Queue: Daco Harkes <[email protected]>

Author: dcharkes

sdk/lib/ffi/struct.dart

@@ -62,5 +62,6 @@ abstract class Struct extends NativeType {
 /// Extension on [Struct] specialized for its subtypes.
 extension StructAddressOf<T extends Struct> on T {
   /// Returns the address backing the reference.
+  @Deprecated('Hold on to the pointer backing a struct instead.')
   Pointer<T> get addressOf => _addressOf as Pointer<T>;
 }