Add field allocf_ud to replace current ud.

Since some use it as `mrb_state` associated user data.

include/mruby.h

@@ -112,6 +112,7 @@ typedef struct mrb_state {
   struct mrb_jmpbuf *jmp;
 
   mrb_allocf allocf;                      /* memory allocation function */
+  void *allocf_ud;                        /* auxiliary data of allocf */
 
   struct mrb_context *c;
   struct mrb_context *root_c;