write free function

[turuslan]

• (Refactor read write some #206)
• replace Writer::write with write/writeReturnSize
  • writeReturnSize doesn't change callback type to reduce changes to review
    • old usages may be simplified/refactored later in other pr's
• remove Writer::write
  • Writer::write can't get shared_ptr from this
    • enable_shared_from_this would conflict (Reader, Writer, user class)

[xDimon]

Why should we create span explicitly?

[turuslan]

They moved buffer into lambda, so created span before move.
(This old code may be unstable, std::function/boost::asio::io_context may copy lambda and duplicate/invalidate buffer, sptr<lambda>/sptr<buffer> would be better; #226)

buffer != (null, 0)
span = span(buffer)
lambda.buffer = move(buffer)
buffer == (null, 0)
write(span, lambda)