std::rt: Some I/O cleanup
std::rt: Add libuv bindings for getaddrinfo
std::rt: Add get_host_addresses function

This is a very simplistic method for host name resolution. It converts
a host name to a vector of IP addresses. Should be enough to get started.
auto merge of #9000 : brson/rust/dns, r=anasazi

This exposes a very simple function for resolving host names. There's a lot more that needs to be done, but this is probably enough for servo to get started connecting to real websites again.