std.net: use send/recv for streams, support vectorized network io on windows

[truemedian]

The primary goal of this change is to use send and recv for network streams, this also enables passing per-message flags like MSG_NOSIGNAL instead of installing a SIGPIPE handler.

• Alongside using recv and send, windows now uses WSARecvFrom and WSASendTo for readv and writev. This enables vectorized network I/O on windows.

Introduces a mostly platform agnostic iovec as std.net.iovec and std.net.iovec_const. This provides a singular interface to vectorized network I/O on windows and posix.

• The only problem this introduces is windows using u32 for lengths and posix using usize. This requires any code currently using usize to @intCast into the length field.

Removes std.os.windows.ws2_32.WSARecvMsg, as it is not present in ws2_32.dll and must be fetched via WSAIoctl.

• The sendto and recvfrom wrappers around WSASendTo and WSARecvFrom have been removed from std.os.windows. These functions are already provided by ws2_32 and are not needed.

A os-agnostic IoVec struct (re #7699) would likely be more convienent to use, but I could not design an interface that both works well and is reasonable to interact with.

A potential problem this PR introduces is passing WSABUF_const to WSASendTo and WSASend is that this type doesn't actually exist in windows. Windows makes no guarantees that the buffers will not be modified.

This enables the "general client/server API coverage" test on windows. The comment on the skip claims that the test was never passing, however during my tests the test passed 100% of the time.

Depends on #19768.

[daurnimator]

For what it's worth, the const-ness for IoSlice gets more complicated
with e.g. sendmsg where we need msghdr and msghdr_const
and then again with e.g. sendmmsg where we have e.g. mmsghdr_const vs msghdr_const

[truemedian]

I decided to avoid writing definitions for msghdr and mmsghdr because nothing in std.net needs them (right now, they are necessary for sending flags with iovecs, but that can be easily hidden). I'm not sure a cross platform definition of msghdr is even necessary for TCP because WSASend/WSARecv already do vectorized IO and don't enable any extra features. And outside of Windows, []IoSlice can be ptrCast into an []iovec because they're guaranteed to have the same representation