Make RelativeName::from_slice const fn

[WhyNotHugo]

Allow defining relative names which are validated resolved at compile time. This is mostly useful for infallible functions which need to use a RelativeName which is not taken as input.

This is also useful for traits where each implementation needs to define
a specific RelativeName. E.g.:

trait DiscoverableService {
    const RELATIVE_NAME: RelativeName<u8>;
}

Example (also included in test module):

const VALID_NAME: &RelativeName<[u8]> =
    match RelativeName::from_slice(b"\x03www\x07example") {
        Ok(name) => name,
        Err(_) => panic!("VALID_NAME failed at compile time"),
    };

A few other functions in the call tree of from_slice had to be made const fn too, some with small changes (like match incited of ?, or inlining .into() calls for errors).

[WhyNotHugo]

This is also usable for patterns like:

  pub struct DiscoverableService {
      pub relative_domain: &'static RelativeName<[u8]>,
      pub scheme: Scheme,
      pub well_known_path: &'static str,
      pub default_port: u16,
      pub access_field: &'static str,
      pub display_name: &'static str,
  }

  impl DiscoverableService {
      pub const CALDAVS: Self = Self {
          relative_domain: match RelativeName::from_slice(b"\x08_caldavs\x04_tcp") { … },
          scheme: Scheme::HTTPS,
          well_known_path: "/.well-known/caldav",
          default_port: 443,
          access_field: "calendar-access",
          display_name: "caldavs",
      };
..