impl: switch most atomic ops to Relaxed ordering

These all seem pretty straight-forward. Compared with #2706, I dropped
the changes to the atomic orderings used in `ignore` because I haven't
had time to think through that carefully. But the ops in this PR seem
fine.

Closes #2706

crates/ignore/src/lib.rs

@@ -527,7 +527,7 @@ mod tests {
 
             let tmpdir = env::temp_dir();
             for _ in 0..TRIES {
-                let count = COUNTER.fetch_add(1, Ordering::SeqCst);
+                let count = COUNTER.fetch_add(1, Ordering::Relaxed);
                 let path = tmpdir.join("rust-ignore").join(count.to_string());
                 if path.is_dir() {
                     continue;