Look up the organizational (registrable) domain.
The caller is expected to pass an already-normalized domain (typically
via normalize_domain); the returned slice borrows from s.
Normalize a domain for use with domain_str / suffix_str:
strip a single trailing dot (FQDN form) and lowercase any ASCII upper
bytes. Borrows when the input is already normalized; only allocates
when uppercase bytes are present.
Look up the public suffix.
The caller is expected to pass an already-normalized domain (typically
via normalize_domain); the returned slice borrows from s.