Make the ffi crate no_std

[Person-93]

I've added no_std to the python-ffi crate.

Most of the changes in this PR are a search-and-replace from std:: to core::. The only other change I've made is in the datetime file. It used the Once type from std. I've updated it to use AtomicBool instead.

This is the first PR from this converstaion: #5856 (comment)

[codspeed-hq]

Merging this PR will improve performance by 13.7%

⚡ 1 improved benchmark
✅ 104 untouched benchmarks
⏩ 1 skipped benchmark¹

Performance Changes

	Benchmark	BASE	HEAD	Efficiency
⚡	bench_pyclass_create	4.6 µs	4.1 µs	+13.7%

---

_{Comparing Person-93:no_std_ffi (48b06f9) with main (f57bda7)}

Footnotes

1. 1 benchmark was skipped, so the baseline result was used instead. If it was deleted from the codebase, click here and archive it to remove it from the performance reports. ↩

[Person-93]

The ffi crate currently uses std::thread::park to work around the interpreter calling pthread_exit for python versions older than 3.14.

pyo3/pyo3-ffi/src/pystate.rs

Lines 82 to 92 in f57bda7

	#[cfg(not(any(Py_3_14, target_arch = "wasm32")))]
	struct HangThread;
	#[cfg(not(any(Py_3_14, target_arch = "wasm32")))]
	impl Drop for HangThread {
	fn drop(&mut self) {
	loop {
	std::thread::park(); // Block forever.
	}
	}
	}

I can think of two ways to handle this.

• move the usage into the main crate
• use a feature-flag to conditionally use a separate crate that can sleep in no_std

The first way is technically a breaking change if anyone is using the PyGILState_Ensure method from the ffi crate as they would have to deal with pinned stack objects some other way, but I think it's simpler, so I will go with that one unless you object.