Resets pos, last, mark member of |buf| to buf.begin.
This points to the beginning of the buffer. The effective range of buffer is [begin, end).
This points to the memory one byte beyond the end of the buffer.
The position indicator for effective one beyond of the end of the buffer. last <= end must be hold.
Mark arbitrary position in buffer [begin, end)
The position indicator for effective start of the buffer. pos <= last must be hold.
true if the allocator must be secure
true if the memory much be manually freed. use_secure_mem can be false if this is true