Calculate offsets for weakreflist and dict in PyCell.

[sebpuetz]

As demonstrated by #1022, depending on the order of structmembers to calculate offsets in bytes is fragile. This PR adds a dependency to memoffset which provides an interface to find offsets into structs in a more robust manner.

Caveat: This macro is technically unsound, see Gilnaa/memoffset#24, so I'm marking this as a draft for now.

This PR moves the calcualtion of class dict and weakreflist offsets to PyCell rather than initialize_type_object.

---

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[sebpuetz]

An alternative implementation that makes maintenance easier without pulling in unsoundness:

#[repr(C)]
pub struct PyCell<T: PyClass> {
    inner: PyCellInner<T>,
    thread_checker: T::ThreadChecker,
    // DO NOT CHANGE THE ORDER OF THESE FIELDS WITHOUT CHANGING PyCell::dict_offset()
    // AND PyCell::weakref_offset()
    dict: T::Dict,
    weakref: T::WeakRef,
}

impl<T: PyClass> PyCell<T> {
    /// Get the offset of the dictionary from the start of the struct in bytes.
    pub(crate) fn dict_offset() -> usize {
        mem::size_of::<Self>() - mem::size_of::<T::Dict>() - mem::size_of::<T::WeakRef>()
    }

    /// Get the offset of the weakref list from the start of the struct in bytes.
    pub(crate) fn weakref_offset() -> usize {
        mem::size_of::<Self>() - mem::size_of::<T::WeakRef>()
    }
}

[sebpuetz]

@kngwyu

[kngwyu]

How about moving this if statement to the PyCell?
I don't like that dict_offset can return a meaningless value even if it doesn't have a dict.

[sebpuetz]

Good catch, I PyCell::*_offset now returns Option<usize>

[kngwyu]

Thanks!

[sebpuetz]

Do you want to add a commit with the test case you wrote for #1061?

[kngwyu]

Do you want to add a commit with the test case you wrote for #1061?

Yeah, could you please copy-paste it to this branch?

[sebpuetz]

Should be good to go if CI passes

[kngwyu]

Thanks!