lua: Add ability to pass lua functions directly to vimL

src/nvim/eval/typval.h

@@ -271,6 +271,11 @@ typedef struct {
 /// Number of fixed variables used for arguments
 #define FIXVAR_CNT 12
 
+/// Callback interface for C function reference
+typedef int (*cfunc_T)(int argcount, typval_T *argvars, typval_T *rettv, void *state);  // NOLINT
+/// Callback to clear cfunc_T
+typedef void (*cfunc_free_T)(void *state);
+
 // Structure to hold info for a function that is currently being executed.
 typedef struct funccall_S funccall_T;
 
@@ -307,6 +312,10 @@ struct ufunc {
   garray_T     uf_lines;         ///< function lines
   int          uf_profiling;     ///< true when func is being profiled
   int          uf_prof_initialized;
+  // Managing cfuncs
+  cfunc_T      uf_cb;            ///< C function extension callback
+  cfunc_free_T uf_cb_free;       ///< C function extesion free callback
+  void        *uf_cb_state;      ///< State of C function extension.
   // Profiling the function as a whole.
   int          uf_tm_count;      ///< nr of calls
   proftime_T   uf_tm_total;      ///< time spent in function + children