Build a Fixed-Size Pool

Implement an opaque fixed-block pool: ``c typedef struct pool pool_t; pool_t *pool_create(size_t blocks, size_t block_size); void *pool_get(pool_t *p); void pool_put(pool_t *p, void *blk); ` pool_get returns a block or NULL when exhausted; pool_put` returns a block to the free list; a just-returned block must be handed out again first (LIFO).