Let ns_resolve_async also parse literal addresses

This commit enables us to reuse the same ns_resolve_async function used for resolving hostnames
to parse literal (numeric) ipv4 and ipv6 addresses, which greatly simplifies the (WIP, see next PR) control flow in the (async) `ns_parse_address`.

DIFFBASE=#141

(DO NOT MERGE pull requests with a DIFFBASE label in the description)