zone.lisp: General support for unknown record types.

zone.lisp: General support for unknown record types.

There's now a `zone-record-rrdata' protocol for zone record types to
serialize themselves in the correct format, and implementations for all
of the record types currently emitted.

There's also a `zone-write-raw-rrdata' protocol for writing these out
when the format doesn't have a better plan, which is used by a (new)
default method on `zone-write-record'.