Missing migration: `SpawnableList` migration does not mention `move_as_ptr`

[janhohenheim]

What problem does this solve or what need does it fill?

While migrating BEI's SpawnableList impls, this pattern came up often:

// 0.16
let foo = something;
foo.spawn(world, entity);

But foo is now required to be MovingPtr! I first built it manually using unsafe until I stumbled on move_as_ptr.
the correct way is now:

// 0.17
let foo = something;
move_as_ptr!(foo);
SpawnableList::spawn(foo, world, entity);