Use std::set to track the layer-wide feature attribute types.

Track them during parsing, not tiling.  Remove the old string pool code.

pool.hpp

@@ -1,28 +1,3 @@
-struct pool_val {
-	const char *s;
-	int type;
-	int n;
-
-	struct pool_val *left;
-	struct pool_val *right;
-
-	struct pool_val *next;
-};
-
-struct pool {
-	struct pool_val **vals;
-
-	struct pool_val *head;
-	struct pool_val *tail;
-	int n;
-};
-
-struct pool_val *pool(struct pool *p, const char *s, int type);
-void pool_free(struct pool *p);
-void pool_free_strings(struct pool *p);
-void pool_init(struct pool *p, int n);
-int is_pooled(struct pool *p, const char *s, int type);
-
 struct stringpool {
 	long long left;
 	long long right;