Ensure the exercises use the canonical format

Add the check-exercises.py tool in the new tools directory.  It is used
to check that the exercises are correctly formatted, printing on stderr
the invalid ones and the diff in the unified format.

Update the exercises that don't use the canonical zig fmt format.

Update some patches that cause the generated zig file to be incorrectly
formatted.

exercises/083_anonymous_lists.zig

@@ -18,8 +18,8 @@ pub fn main() void {
     //
     // Don't change this part:
     //
-    //     = .{'h', 'e', 'l', 'l', 'o'};
+    //     = .{ 'h', 'e', 'l', 'l', 'o' };
     //
-    const hello = .{'h', 'e', 'l', 'l', 'o'};
+    const hello = .{ 'h', 'e', 'l', 'l', 'o' };
     print("I say {s}!\n", .{hello});
 }