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.

tok.h

@@ -4,6 +4,8 @@
 #include <stdint.h>
 #include <unistd.h>
 
+#include "util.h"
+
 typedef struct Type {
 	enum {
 		TypeVoid = 0,
@@ -87,10 +89,7 @@ typedef struct TokListItem {
 
 typedef struct TokList {
 	TokListItem *begin, *end;
-	TokListItem *mempools[32]; /* few mallocs, no copying => much speed */
-	size_t mempool_sizes[32];
-	size_t curr_mempool_cap;
-	size_t curr_mempool;
+	Pool *p;
 } TokList;
 
 void toklist_init(TokList *l);