Upgrade to rustfmt 0.8.0.

Lots of changes this time.

Worked around what looks like a rustfmt bug in parse_inst_operands where
a large match was nested inside Ok().

lib/cretonne/src/partition_slice.rs

@@ -35,7 +35,10 @@ mod tests {
 
     fn check(x: &[u32], want: &[u32]) {
         assert_eq!(x.len(), want.len());
-        let want_count = want.iter().cloned().filter(|&x| x % 10 == 0).count();
+        let want_count = want.iter()
+            .cloned()
+            .filter(|&x| x % 10 == 0)
+            .count();
         let mut v = Vec::new();
         v.extend(x.iter().cloned());
         let count = partition_slice(&mut v[..], |&x| x % 10 == 0);