patch 8.2.3725: cannot use a lambda for 'completefunc' and 'omnifunc'

Problem:    Cannot use a lambda for 'completefunc' and 'omnifunc'.
Solution:   Implement lambda support. (Yegappan Lakshmanan, closes #9257)
diff --git a/src/structs.h b/src/structs.h
index 48a531b..be444e3 100644
--- a/src/structs.h
+++ b/src/structs.h
@@ -2876,7 +2876,9 @@
 #endif
 #ifdef FEAT_COMPL_FUNC
     char_u	*b_p_cfu;	// 'completefunc'
+    callback_T	b_cfu_cb;	// 'completefunc' callback
     char_u	*b_p_ofu;	// 'omnifunc'
+    callback_T	b_ofu_cb;	// 'omnifunc' callback
 #endif
 #ifdef FEAT_EVAL
     char_u	*b_p_tfu;	// 'tagfunc' option value
@@ -2982,6 +2984,7 @@
     char_u	*b_p_tsr;	// 'thesaurus' local value
 #ifdef FEAT_COMPL_FUNC
     char_u	*b_p_tsrfu;	// 'thesaurusfunc' local value
+    callback_T	b_tsrfu_cb;	// 'thesaurusfunc' callback
 #endif
     long	b_p_ul;		// 'undolevels' local value
 #ifdef FEAT_PERSISTENT_UNDO