Mostly confine assumptions about what token_t is to misc.c

The token_t things like WD* are presently longs and will someday be
char[6].  By introducing some trivial functions - wordeq(),
wordempty(), and wordclear() - we mostly hide the difference.

All runtime knowledge about packing now lives only in misc.c and the
list of magic WORD_* defines in advent.h.  Outside this, literals are
now accessed through #define names that could expand to either longs
or strings.

Still to be done: WD* values are sometiimes compated to zero in
ways implying they can be negative. Must figure out wat thus means.

misc.c

@@ -50,6 +50,23 @@ void packed_to_token(long packed, char token[6])
     }
 }
 
+/* Hide the fact that wods are corrently packed longs */
+
+bool wordeq(token_t a, token_t b)
+{
+    return a == b;
+}
+
+bool wordempty(token_t a)
+{
+    return a == 0;
+}
+
+void wordclear(token_t *v)
+{
+    *v = 0;
+}
+
 /*  I/O routines (SPEAK, PSPEAK, RSPEAK, SETPRM, GETIN, YES) */
 
 void speak(const char* msg)