Implement basic IPv6 address parsing support

Summary: Ref T11939. I'll hold this until after the release cut. We still need CIDR support (although that looks straightforward) and then testing, which should be only slightly painful since AWS supports IPv6 with some caveats now.

Test Plan: Added unit tests; ran unit tests.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T11939

Differential Revision: https://secure.phabricator.com/D16973