add assignment operator and unify memory pools

The unification of memory pools also fixed some memory leaks and
hopefully reduced the mallocs of identifier strings significantly by
giving them the same pool as the token stream.

map.h

@@ -26,6 +26,6 @@ void map_term(Map *m);
 bool map_insert(Map *m, const char *key, const void *val);
 /* Returns true if the key was found, returns false if it wasn't found.
  * out_val may be set to NULL. */
-bool map_get(Map *m, const char *key, void *out_val);
+bool map_get(const Map *m, const char *key, void *out_val);
 
 #endif /* MAP_H */