replace %defer with errdefer

See #632

now we have 1 less sigil

test/cases/defer.zig

@@ -8,7 +8,7 @@ error FalseNotAllowed;
 fn runSomeErrorDefers(x: bool) -> %bool {
     index = 0;
     defer {result[index] = 'a'; index += 1;}
-    %defer {result[index] = 'b'; index += 1;}
+    errdefer {result[index] = 'b'; index += 1;}
     defer {result[index] = 'c'; index += 1;}
     return if (x) x else error.FalseNotAllowed;
 }