API: Refactor: Return handles instead of indexes

- Define specialized arrays for each remote object type
- Implement msgpack_rpc functions for dealing with the new types
- Refactor all functions dealing with buffers, windows and tabpages to
  return/accept handles instead of list indexes.

src/nvim/api/tabpage.c

@@ -6,10 +6,11 @@
 #include "nvim/api/vim.h"
 #include "nvim/api/private/defs.h"
 #include "nvim/api/private/helpers.h"
+#include "nvim/memory.h"
 
-Integer tabpage_get_window_count(Tabpage tabpage, Error *err)
+WindowArray tabpage_get_windows(Tabpage tabpage, Error *err)
 {
-  Integer rv = 0;
+  WindowArray rv = {.size = 0};
   tabpage_T *tab = find_tab(tabpage, err);
 
   if (!tab) {
@@ -23,7 +24,17 @@ Integer tabpage_get_window_count(Tabpage tabpage, Error *err)
     if (tp != tab) {
       break;
     }
-    rv++;
+    rv.size++;
+  }
+
+  rv.items = xmalloc(sizeof(Window) * rv.size);
+  size_t i = 0;
+
+  FOR_ALL_TAB_WINDOWS(tp, wp) {
+    if (tp != tab) {
+      break;
+    }
+    rv.items[i++] = wp->handle;
   }
 
   return rv;
@@ -67,13 +78,11 @@ Window tabpage_get_window(Tabpage tabpage, Error *err)
   } else {
     tabpage_T *tp;
     win_T *wp;
-    rv = 1;
 
     FOR_ALL_TAB_WINDOWS(tp, wp) {
       if (tp == tab && wp == tab->tp_curwin) {
-        return rv;
+        return wp->handle;
       }
-      rv++;
     }
     // There should always be a current window for a tabpage
     abort();