Replace all direct pointers in rules with typedef.

Moving towards true C++ type safety by retyping all raw pointers. The
typedefs will eventually be replaced with managed pointers.

src/engine/function.h

@@ -1,4 +1,5 @@
 /*
+ *  Copyright 2022 René Ferdinand Rivera Morell
  *  Copyright 2011 Steven Watanabe
  *  Distributed under the Boost Software License, Version 1.0.
  *  (See accompanying file LICENSE.txt or https://www.bfgroup.xyz/b2/LICENSE.txt)
@@ -17,6 +18,9 @@
 typedef struct _function FUNCTION;
 typedef struct _stack STACK;
 
+typedef FUNCTION* function_ptr;
+typedef STACK* stack_ptr;
+
 STACK * stack_global( void );
 void stack_push( STACK * s, LIST * l );
 LIST * stack_pop( STACK * s );