Stabilize Ipv6Addr::is_unique_local and Ipv6Addr::is_unicast_link_local

[umgefahren]

Make Ipv6Addr::is_unique_local and Ipv6Addr::is_unicast_link_local stable (+const).

Newly stable API:

impl Ipv6Addr {
	// Newly stable under `ipv6_is_unique_local`
	const fn is_unique_local(&self) -> bool;

	// Newly stable under `ipv6_is_unique_local`
	const fn is_unicast_link_local(&self) -> bool;
}

These stabilise a subset of the following tracking issue:

• Tracking issue for Ipv{4,6}Addr convenience methods #27709

I have looked and could not find any issues with is_unique_local and is_unicast_link_local. There is a well received comment calling for stabilisation of the latter function.

Both functions are well defined and consistent with implementations in other languages:

• Go
• Python
• Ruby (unique local)
• Ruby (unicast link local)

cc implementor @little-dude
(I can't find the original PR for is_unqiue_local)

r? libs-api
@rustbot label +T-libs-api +needs-fcp