Returns a mutable buffer sequence representing writable bytes.
mutable_buffers_type prepare( std::size_t n);
Returns a mutable buffer sequence representing the writable bytes containing
n bytes of storage.
This function does not allocate memory. Instead, the storage comes from
the underlying mutable buffer sequence. All buffer sequences previously
prepare are invalidated. Buffer
sequences previously obtained using
data remain valid.
The desired number of bytes in the returned buffer sequence.