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/regalloc/allocatable_set.rs

@@ -118,21 +118,21 @@ mod tests {
 
     // Register classes for testing.
     const GPR: RegClass = &RegClassData {
-        name: "GPR",
-        index: 0,
-        width: 1,
-        first: 28,
-        subclasses: 0,
-        mask: [0xf0000000, 0x0000000f, 0],
-    };
+                               name: "GPR",
+                               index: 0,
+                               width: 1,
+                               first: 28,
+                               subclasses: 0,
+                               mask: [0xf0000000, 0x0000000f, 0],
+                           };
     const DPR: RegClass = &RegClassData {
-        name: "DPR",
-        index: 0,
-        width: 2,
-        first: 28,
-        subclasses: 0,
-        mask: [0x50000000, 0x0000000a, 0],
-    };
+                               name: "DPR",
+                               index: 0,
+                               width: 2,
+                               first: 28,
+                               subclasses: 0,
+                               mask: [0x50000000, 0x0000000a, 0],
+                           };
 
     #[test]
     fn put_and_take() {