Change most functions in `ratelimits` to use full ACMEIdentifier(s) as
arguments, instead of using their values as strings. This makes the
plumbing from other packages more consistent, and allows us to:
Rename `FQDNsToETLDsPlusOne` to `coveringIdentifiers` and handle IP
identifiers, parsing IPv6 addresses into their covering /64 prefixes for
CertificatesPerDomain[PerAccount] bucket keys.
Port improved IP/CIDR validation logic to NewRegistrationsPerIPAddress &
PerIPv6Range.
Rename `domain` parts of bucket keys to either `identValue` or
`domainOrCIDR`.
Rename other internal functions to clarify that they now handle
identifier values, not just domains.
Add the new reserved IPv6 address range from RFC 9780.
For deployability, don't (yet) rename rate limits themselves; and
because it remains the name of the database table, preserve the term
`fqdnSets`.
Fixes#8223
Part of #7311
48d5ad3c19