Constify `std::net::IpAddr`, `Ipv4Addr`, and `Ipv6Addr`

[CDirkx]

Due to recent stabilizations of const integer arithmetic and const control flow, all methods of types std::net::IpAddr, Ipv4Addr, and Ipv6Addr can be made const. This issue provides an overview of all methods and steps that need to be taken:

Ipv4Addr

Methods already stable const:

• new
• is_unspecified

Methods unstable const by PR#76142 (const_ipv4):

• octets
• is_loopback
• is_private
• is_link_local
• is_shared (unstable)
• is_ietf_protocol_assignment (unstable)
• is_benchmarking (unstable)
• is_multicast
• is_documentation
• is_global (unstable)
• is_reserved (unstable)
• is_broadcast
• to_ipv6_compatible (deprecated?)
• to_ipv6_mapped

None of these methods depends on an unstable feature, and could all thus be made stable const.

Ipv6Addr

Methods already stable const:

• new (uses #[allow_internal_unstable(const_fn_transmute)])
• octets

Ipv6Addr::segments has been made unstable const in PR#76206 (const_ipv6), together with all dependent methods:

• segments
• is_unspecified
• is_loopback
• is_global (unstable)
• is_unique_local (unstable)
• is_unicast_link_local_strict (unstable)
• is_documentation (unstable)
• multicast_scope (unstable)
• is_multicast
• to_ipv4_mapped (unstable)
• to_ipv4

All of these methods depend on segments, and thus upon the unstable feature const_fn_transmute, however the transmute is only used for better code generation, the operations could be rewritten equivalently using stable const code.

IpAddr

The following methods are stable const:

• is_ip4
• is_ipv6

Implemented in PR#76226.

With the relevant methods of Ipv4Addr and Ipv6Addr made const, the following were also made unstable const in PR#76304:

• is_documentation (unstable)
• is_global (unstable)
• is_loopback
• is_multicast
• is_unspecified