api: unify buffer numbers and window ids with handles

also allow handle==0 meaning curbuf/curwin/curtab

src/nvim/terminal.c

@@ -127,7 +127,7 @@ struct terminal {
   // we can't store a direct reference to the buffer because the
   // refresh_timer_cb may be called after the buffer was freed, and there's
   // no way to know if the memory was reused.
-  uint64_t buf_handle;
+  handle_T buf_handle;
   // program exited
   bool closed, destroy;
   // some vterm properties