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
flat_static_buffer::prepare are invalidated.
The number of bytes to append. If this number is greater than the number of writable bytes, all writable bytes are appended.