std: make address resolution weirdness local to SGX

[joboet]

Currently, the implementations of TcpStream::connect and its cousins take an io::Result<&SocketAddr> as argument, which is very weird, as most of them then ?-try the result immediately to access the actual address. This weirdness is however necessitated by a peculiarity of the SGX networking implementation:

SGX doesn't support DNS resolution but rather accepts hostnames in the same place as socket addresses. So, to make e.g.

TcpStream::connect("example.com:80")`

work, the DNS lookup returns a special error (NonIpSockAddr) instead, which contains the hostname being looked up. When .to_socket_addrs() fails, the each_addr function used to select an address will pass the error to the inner TcpStream::connect implementation, which in SGX's case will inspect the error and try recover the hostname from it. If
that succeeds, it continues with the found hostname.

This is pretty obviously a terrible hack and leads to buggy code (for instance, when users use the result of .to_socket_addrs() in their own ToSocketAddrs implementation to select from a list of possible URLs, the only URL used will be that of the last item tried). Still, without changes to the SGX usercall ABI, it cannot be avoided.

Therefore, this PR aims to minimise the impact of that weirdness and remove it from all non-SGX platforms. The inner TcpStream::connect, et al. functions now receive the ToSocketAddrs type directly and call each_addr (which is moved to sys::net::connection) themselves. On SGX, the implementation uses a special each_addr which contains the whole pass-hostname-through-error hack.

As well as making the code cleaner, this also opens up the possibility of reusing newly created sockets even if a connection request fails – but I've left that for another PR.

CC @raoulstrackx

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[joboet]

I've moved the whole platform-matching into this new file so there's a place to put each_addr.

[joboet]

If you were wondering why this is its own module, it's to facilitate #135141, which has a different platform distribution.