[ffi] Option object missing definition in .hpp

[samansmink]

Describe the bug

The current (v0.7.0) release of kernel will produce an ffi header (.hpp) like:

...
template<typename T = void>
struct Option;
...
Option<Handle<SharedExpression>> get_transform_for_row(uintptr_t row,
                                                       const CTransforms *transforms);
...

This means that I can't use the result returned form the get_transform_for_row but it also leads to a compile issue because I can't use the struct trick thing to workaround from #451

Workaround for now is really simple though: i just comment out the get_transform_for_row function

To Reproduce

build with cargo build --package delta_kernel_ffi --workspace --all-features

Expected behavior

The get_transform_for_row function should return an object that has a definition to be able to use it and allow for me to use the hacky workaround for #451

[scovich]

Ugh, this is that old cbindgen gap again -- it's not smart enough to collapse Option<T> when T is a transparent struct wrapping NonNull. The fix I contributed way back in November has unfortunately languished without reviews: mozilla/cbindgen#1029