implement IPv6 scoped addresses (RFC4007)

[foxxx0]

In order to translate interface names in such scoped addresses the
required LibC binding to if_nametoindex() has been added.
This method obviously only works for interfaces (devices) that are
actually present on the system.

The binding for the reverse operation if_indextoname() has also been
added, although its usage is a bit more cumbersome due to LibC::Char*
buffer handling. The necessary buffer length has been placed into the
constant LibC::IF_NAMESIZE, which appears to be 16u8 on unix-like
systems and 257u16 on windows.
This could potentially be reworked via a preprocessor block at
compile-time as indicated by some folks over on discord, I currently do
not know how to achieve that though.

Scoped identifiers are only valid for link-local (fe80::) addresses, e.g. fe80::1%eth0

References:

• https://datatracker.ietf.org/doc/html/rfc4007

TODO (to be resolved during the PR discussion):

• discuss the preprocessor option for retrieving the constant value IF_NAMESIZE (No clue how do that, haven't found any event of this constant ever changing in the past, might not be worth the trouble?)
• clarify whether scope_id parsing should raise ArgumentError or Socket::Error
• clarify whether Socket::Address spec #scope_id looks up interface name by index should remain in the Socket::Address spec, since it only calls the LibC binding

I'm happy to adjust/rebase if needed based on feedback or other merges into master in the mean time.

Fixes #15264 .

[straight-shoota]

Could you please open an issue as a feature request first? We ask this to separate high-level discussion of the feature (in the issue discussion) from details of this specific PR (PR discussion). Ref: https://github.com/crystal-lang/crystal/blob/master/CONTRIBUTING.md#using-the-issue-tracker

I'm marking this PR as draft for now.

[foxxx0]

ah well, shame on me.

I've just created #15264 now to discuss the feature itself.