All IPv6 addresses would be considered globally routeable, even though
the Rust standard library offers convenience functions to check for
loopback, link-local addresses etc.
Improve checks for IPv6 routeability to catch the most obvious cases of
local or unspecified addresses.
Refactor the check for IPv4 routeability to be more readable and refer
to RFCs, IANA lists, and Rust stabilization tracking issues as
appropriate.
37d4ae4a9f