std::net: lookup_host_as_family

[stepancheg]

Sometimes you need to resolve address as specific address family.

• enum IpAddrFamily
• add simple test of lookup_host
• lookup_host_as_family function that takes IpAddrFamily as second parameter

[rust-highfive]

r? @nikomatsakis

(rust_highfive has picked a reviewer for you, use r? to override)

[alexcrichton]

Sorry for taking awhile to get back to this, but I'm going to close this for the same reasons as #23161. These sorts of additions need to go through an RFC first, and I think this would be a pretty good candidate for starting a wishlist issue in general for std::net. Thanks for the PR though!