Make getaddrinfo pluggable

[njsmith]

APIs that perform name lookups should take an argument (defaulting to trio.socket.getaddrinfo) specifying the function to use for looking up names. This is useful for testing, and it's useful for folks who might want to use some other name lookup code (e.g. an async dns library like pycares).

[njsmith]

The getaddrinfo hook added in #253 provides this (though in a different way than suggested above).