api/msgpack-rpc: Remove specialized array types

Specialized array types(BufferArray, WindowArray, etc) were added to the API for
two main reasons:

- msgpack used to lack a way of serializing appliaction-specific types and there
  was no obvious way of making an API function accept/return arrays of custom
  objects such as buffers(which are represented as integers, so clients didn't
  have a way to distinguish from normal numbers)
- Let clients in statically-typed languages that support generics have a better
  typed API

With msgpack 2.0 EXT type the first item is no longer a factor and this commit
starts by removing the specialized array types. The second item will be
addressed in the future by making the API metadata return extra useful
information for statically-typed languages.

src/nvim/api/tabpage.c

@@ -13,9 +13,9 @@
 /// @param tabpage The tabpage
 /// @param[out] err Details of an error that may have occurred
 /// @return The number of windows in `tabpage`
-WindowArray tabpage_get_windows(Tabpage tabpage, Error *err)
+Array tabpage_get_windows(Tabpage tabpage, Error *err)
 {
-  WindowArray rv = ARRAY_DICT_INIT;
+  Array rv = ARRAY_DICT_INIT;
   tabpage_T *tab = find_tab_by_handle(tabpage, err);
 
   if (!tab) {
@@ -32,14 +32,14 @@ WindowArray tabpage_get_windows(Tabpage tabpage, Error *err)
     rv.size++;
   }
 
-  rv.items = xmalloc(sizeof(Window) * rv.size);
+  rv.items = xmalloc(sizeof(Object) * rv.size);
   size_t i = 0;
 
   FOR_ALL_TAB_WINDOWS(tp, wp) {
     if (tp != tab) {
       break;
     }
-    rv.items[i++] = wp->handle;
+    rv.items[i++] = WINDOW_OBJ(wp->handle);
   }
 
   return rv;