build: enable -Wshadow

Note about shada.c:
- shada_read_next_item_start was intentionally shadowing `unpacked` and
  `i` because many of the macros (e.g. ADDITIONAL_KEY) implicitly
  depended on those variable names.
- Macros were changed to parameterize `unpacked` (but not `i`). Macros
  like CLEAR_GA_AND_ERROR_OUT do control-flow (goto), so any other
  approach is messy.

src/nvim/msgpack_rpc/channel.c

@@ -495,8 +495,8 @@ static void broadcast_event(const char *name, Array args)
                                       kv_size(subscribed));
 
   for (size_t i = 0; i < kv_size(subscribed); i++) {
-    Channel *channel = kv_A(subscribed, i);
-    channel_write(channel, buffer);
+    Channel *c = kv_A(subscribed, i);
+    channel_write(c, buffer);
   }
 
 end: