Append writable bytes to the readable bytes.
void commit( std::size_t n);
Appends n bytes from the start of the writable bytes to the end of the
readable bytes. The remainder of the writable bytes are discarded. If n
is greater than the number of writable bytes, all writable bytes are appended
to the readable bytes. All buffers sequences previously obtained using
prepare become invalid.
The number of bytes to append. If this number is greater than the number of writable bytes, all writable bytes are appended.