Construct a buffer from a span of bytes.

template<
    template< typename, std::size_t > class Span,
    typename T,
    std::size_t Extent>
const_buffer(
    const Span< T, Extent > & span,
    constraint_t< sizeof(T)==1, defaulted_constraint >  = defaulted_constraint(),
    constraint_t< is_same< decltype(span.subspan(0, 0)), Span< T, static_cast< std::size_t >(-1)> >::value, defaulted_constraint >  = defaulted_constraint());