EXP: use py check_signals() to catch CTRL-C

[ctb]

Fixes #40.

Per #40 (comment),

so there's a much simpler way via pyo3! I just enabled this in the directsketch plugin:

1. Pass in py /_py from lib to main function

• https://github.com/sourmash-bio/sourmash_plugin_directsketch/blob/main/src/lib.rs#L86-L91
• https://github.com/sourmash-bio/sourmash_plugin_directsketch/blob/main/src/directsketch.rs#L603

2. Include use pyo3::prelude::*; and call py.check_signals in main function:

• https://github.com/sourmash-bio/sourmash_plugin_directsketch/blob/main/src/directsketch.rs#L669

Related issues:

• Closes WIP: add Ctrl-C interrupt #253
• Closes WIP: improve/extend thread management and ctrl-c #276

[ctb]

Main problem right now - can't use py.check_signals() inside a closure 🤔

[bluegenes]

FYI we can use the rust ctrlc crate, see:

sourmash_plugin_branchwater/src/sigcat.rs

Lines 25 to 40 in b1efabd

	static TERMINATE_FLAG: OnceLock<Arc<AtomicBool>> = OnceLock::new();
	static CTRL_HANDLER_INIT: Once = Once::new();
	pub fn setup_ctrlc_handler() -> Arc<AtomicBool> {
	let flag = TERMINATE_FLAG.get_or_init(|| Arc::new(AtomicBool::new(false)));
	CTRL_HANDLER_INIT.call_once(|| {
	let flag_clone = Arc::clone(flag);
	let _ = ctrlc::set_handler(move || {
	eprintln!("Received Ctrl-C, requesting shutdown...");
	flag_clone.store(true, Ordering::SeqCst);
	});
	});
	Arc::clone(flag)
	}

(pass in the cancel_flag anywhere it is needed; check regularly)