Poke.bin_prot X.bin_write_t t x writes x to the beginning of t in binary form
without advancing. You can use X.bin_size_t to tell how long it was.
X.bin_write_t is only allowed to write that portion of the buffer to which you have
access.
('d, 'w) Iobuf.t accessor function manipulating 'a, either writing it to the
iobuf or reading it from the iobuf.