API: Refactor: Implement api/handle module

This module will be used to implement remote management of objects through the
API. Object types to be registered must have a `uint64_t` field named 'handle'.

src/nvim/api/private/handle.h

@@ -0,0 +1,14 @@
+#ifndef NVIM_API_HANDLE_H
+#define NVIM_API_HANDLE_H
+
+#include "nvim/vim.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);
+
+void handle_init(void);
+
+#endif  // NVIM_API_HANDLE_H
+