add pyo3::sync::with_critical_section_ptr

[ngoldbaum]

This adds a public wrapper for the critical section C API that accepts a raw FFI pointer and refactors with_critical_section to use this new function.

While working on #4921 I thought I needed this but later realized I don't. Still, I think it's worth providing both the safe wrapper and unsafe wrapper in the public API, since sometimes you only have a raw pointer and it would be annoying to have to reimplement this every time.

[davidhewitt]

Unless there is a very clear need for an API taking a raw pointer, I would prefer not to have this. In particular, unsafe functions taking closures are problematic because the unsafe easily infects the closure accidentally:

unsafe {
    with_critical_section_ptr(p, || {
        /* unsuspecting user uses unsafe APIs here due to top level unsafe block */
    })
}

The usual resolution is to require a parameter to be unsafely constructed instead, which happens to also fit our existing API:

with_critical_section(
    unsafe { &Borrowed::from_ptr(p) },
    || { /* no unsafe allowed here */ }
)

... so I think if anything, our existing API is preferred.

[ngoldbaum]

Fair enough, you have a better feeling for this sort of thing than I do.