wasi-sockets: Allow implicit binds in 0.3

[badeend]

In 0.2, implicit binds were explicitly forbidden, because that would bypass the concept of network handles.
0.3 doesn't have network handles and sockets are allowed to perform implicit binds again. This affects:

• tcp-socket::listen (documented here)
• udp-socket::connect (documented here)
• udp-socket::send

Eventually, this will also allow us to remove the double bookkeeping like this from wasi-libc.

[alexcrichton]

Thanks for the comprehensive tests and the implementation here, much appreciated!

[badeend]

Interesting. I was under the impression that implicit bind was a well-known universal behavior, but after some more research it appears to be a bit more nuanced than I thought. I've added a comment in the code with my findings.

But practically, Linux & MacOS both successfully ran to completion with:

test p3::p3_sockets_tcp_listen ... ok

So they perform an implicit bind as expected (but not clearly documented). I assume this behavior extrapolates to other *BSD flavors.

But apparently, Windows requires bind to be called before listen. To take away any potential platform-specific differences, I've updated the code to always perform the "implicit" bind ourselves in case the socket wasn't explicitly bound by the guest.

Assuming this actually fixes the CI problems, I'll open a PR for a spec change to add a clarification on the listen method.

@alexcrichton Could you review the changes I just added?

[alexcrichton]

Sounds right to me yeah, and thanks for investigating!