See docs in Writer_intf for what this is for
val next_buf : ensure_capacity:int -> (Core.read_write, Iobuf.seek) Iobuf.tCalled mainly when there isn't enough buffer space left to write a message of ensure_capacity bytes and a new buffer is needed. May also be called in certain other situations even if there's enough space in the previous buffer. Any older buffers will no longer be used after calling this function, so it's legal for a Destination to re-use Iobuf.ts to avoid allocating.
Record that the writer appended a certain number of bytes to the buffer, will be called some time after the bytes are written but before next_buf is called.