mutable_buffers_type prepare( std::size_t n);
Ensures that the output sequence can accommodate
resizing the basic_string object as necessary.
An object of type
mutable_buffers_type that satisfies MutableBufferSequence
requirements, representing basic_string memory at the start of the output
sequence of size
size() + n > max_size().
The returned object is invalidated by any
basic_string member function that modifies the input sequence
or output sequence.