Create a new non-modifiable buffer that represents the given string.
template< typename Elem, typename Traits> const_buffer buffer( basic_string_view< Elem, Traits > data, std::size_t max_size_in_bytes);
A mutable_buffer
value equivalent to:
mutable_buffer( data.size() ? &data[0] : 0, min(data.size() * sizeof(Elem), max_size_in_bytes));