Remove all unnecessary Enter/ExitHandleScope()s in FfiCalls

[ghost]

All FfiCalls internally call Enter- and ExitHandleScope() for all calls that contain handles in the function signature.
This behaviour will no longer be needed for Handle arguments as of 243320, but removing this behaviour is a breaking change since the native function might rely on there being a scope, to e.g. call Dart API, like Dart_NewSendPort.

We need to investigate how we can best get rid of all unnecessary scopes.
This could for instance be:

• Making a breaking change, and have all users migrate their code to manually set up the necessary scopes. Or ..
• Have an opt-in, like @FfiNative<>(.., requiresScope:false).

[dcharkes]

Adding a parameter sounds like a good idea.

We would have to experiment a bit with this a breaking change, to see how it breaks for user code if they don't set up the scope.

• Silently allocating everything in the top scope having a million handles there would be bad.
• A crash would be good.

Also, we might want to warn on returning a handle and having requiresScope:false because that handle would not be freed from the top handle scope. (Or alternatively, we want to free that handle on return, but that would also be a call to a dynamically linked runtime entry.)