Provide a way to create sockets

[rylev]

This is perhaps dependent on resolving #1 and #4, but a large gap in the wasi's capabilities is that there is currently no way to create sockets (i.e. no analogue to accept or connect). There are currently ways to read and write to sockets (sock_send and sock_recv), but without a good way to get a socket fd, these are not terribly useful.

Given that a major early use case for wasi is in the FaaS and "edge computing" space where networking is pretty important, it makes sense for filling out the socket API to be high on the priority list.

[npmccallum]

This is a requirement for us.

[maspe36]

Is there a working spec somewhere for this feature?