std.Io: vectored sendmsg/recvmsg

[psnszsn]

Add support for vectored sendmsg/recvmsg (netSend/netReceive) in std.Io and the Threaded and Kqueue implementations.

The only users of these functions are std.Io.net.Socket, which I adapted to keep its existing API, and std.Io.Threaded.lookupDns

netReceive was somewhat special-cased for lookupDns I think. Instead of each message having a its own data buffer(or buffers for vectored io), it would use a common buffer and partition it for each message. This fits the lookupDns use-case, but is seriously limiting for a general purpose interface. It makes implementing things such as ring buffers impossible.

The changes boil down to:

• IncomingMessage.data is now caller supplied instead of populated by the Io implementation. It can be a single buffer or multiple for vectored operations
• Added separate field for bytes_sent/bytes_received instead of reusing the slice length
• Made OutgoingMessage.address nullable to allow for usage with connected sockets (TCP, Unix) which do not need an address