API: Refactor: Register/unregister created/destroyed buffers

- Add the 'handle' field to `buf_T`
- Add declare/implement functions for registering/unregistering/retrieving
  buffers
- Register/unregister buffers when they are created/destroyed.

src/nvim/api/private/handle.h

@@ -2,13 +2,17 @@
 #define NVIM_API_HANDLE_H
 
 #include "nvim/vim.h"
+#include "nvim/buffer_defs.h"
 
 #define HANDLE_DECLS(type, name)                                              \
   type *handle_get_##name(uint64_t handle);                                   \
   void handle_register_##name(type *name);                                    \
   void handle_unregister_##name(type *name);
 
+HANDLE_DECLS(buf_T, buffer)
+
 void handle_init(void);
 
+
 #endif  // NVIM_API_HANDLE_H