Support rayon-like parallel execution

[nikomatsakis]

Salsa today supports parallelism but in a kind of heavyweight way. It should be possible to easily "map" a query across a long list of inputs using rayon-like parallel processing -- maybe we can even literally use rayon. I haven't put much thought into this. I also want to get rid of the "sendable vs not-sendable" database distinction. Let's just require everything to be sendable and be done with it.

[axelkar]

Can't you currently hook up par_iter here? Would adding + Sync to the dyn work?

salsa/examples/lazy-input/main.rs

Lines 190 to 212 in cfdefc6

	let links = lines
	.filter_map(|path| {
	let relative_path = match path.parse::<PathBuf>() {
	Ok(path) => path,
	Err(err) => {
	Diagnostic::push_error(
	db,
	input,
	Report::new(err).wrap_err(format!("Failed to parse path: {path}")),
	);
	return None;
	}
	};
	let link_path = input.path(db).parent().unwrap().join(relative_path);
	match db.input(link_path) {
	Ok(file) => Some(parse(db, file)),
	Err(err) => {
	Diagnostic::push_error(db, input, err);
	None
	}
	}
	})
	.collect();

Note that I haven't tested this and am not an expert here.