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.