Move the default block to the front of the underlying jump table storage (#5770)

The new api on JumpTableData makese it easy to keep the default label first, and that shrinks the diff in #5731 a bit.

cranelift/reader/src/parser.rs

@@ -1799,7 +1799,7 @@ impl<'a> Parser<'a> {
 
         self.consume();
 
-        Ok(JumpTableData::new(def, data))
+        Ok(JumpTableData::new(def, &data))
     }
 
     // Parse a constant decl.