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.c

@@ -85,7 +85,7 @@ bool map_insert(Map *m, const char *key, const void *val) {
 	return replaced;
 }
 
-bool map_get(Map *m, const char *key, void *out_val) {
+bool map_get(const Map *m, const char *key, void *out_val) {
 	size_t idx = fnv1a32(key, strlen(key)) & (m->cap - 1);
 	for (;;) {
 		if (m->slots[idx].empty)