net.lisp, zone.lisp: Major overhaul for multiple address families.

A lot of internals have changed, and some user-visible features have
been dropped.

  * IP addresses and networks are now captured in CLOS objects, and the
    low-level details of messing with them are handled in generic
    functions which live in their own separate files.

  * `ipnet-pretty' has gone.  Now `ipnet' objects are directly
    printable.

  * `ipnet-changeable-bytes' has gone; there's now
    `ipnet-changeable-bits' instead.

  * `host' and `net' objects now track multiple addresses, so accessing
    them is a bit different.  `net-get-as-ipnet' has gone, replaced by
    `net-parse-to-ipnets'.  Acceptable syntaxes have mostly been
    enhanced, with the ability to control which address families are
    emitted.

  * Slightly painfully, support for DNS lookups has been dropped --
    because SBCL doesn't have a good way of doing IPv6 lookups.

  * The `:cidr-delegation' record parser has gone, and been replaced by
    `:multi', which can be used to achieve the same thing (and a number
    of other special effects besides).

  * For the sake of sanity, the `:a' record parser only produces A
    records.  The new `:addr' parser will produce records for all
    address families associated with its input.