Add NativeFinalizer to go with malloc and calloc

[dcharkes]

Our malloc and calloc allocators use a different allocator on Windows.

This means that devs wanting to use a NativeFinalizer with malloc and calloc need to branch on OS.

Also, devs wanting to use NativeFinalizers with malloc and calloc rely on the specific memory allocator that package:ffi uses under the hood. This means we cannot change the underlying allocator without breaking dependencies.

If we would provide a NativeFinalizer that would free the memory backed by calloc and malloc, we could change the implementation without breaking people. (Assuming they do not lookup their own "free" somewhere.)

cc @timsneath heads up for package:win32
cc @mkustermann we're locked in to CoTaskMemAlloc

[mkustermann]

Would it be sufficient to add a getter to the Allocator interface:

abstract class Allocator {
  Pointer<T> allocate<T extends NativeType>(int byteCount, {int? alignment});
  void free(Pointer<NativeType> pointer);
+  Pointer<NativeFinalizerFunction> get freeFunction;
}

?

[dcharkes]

We would need to make it nullable, because we're not guaranteed that all functions use the same signature.
And Arena should return null, because it is going to free the things itself.

(The freeFunction would then flow into a NativeFinalizer, and would be unpacked again from the NativeFinalizer in the EagerFinalizable. dart-lang/sdk#49906 👌 )

[mkustermann]

As suggested above, the package has now a Allocator.nativeFree that can be used in finalizers (see dart-archive/ffi#203). I think this should close this issue, right?