HTools/Types.hs: minor adjustments to auto-repair types

HTools/Types.hs: minor adjustments to auto-repair types

In particular:

  - make ArHealthy take an optional AutoRepairData; this allows to
    represent the situation where a repair completed successfully, and
    hence there's an associated tag we might want to know about.

  - add an 'arTag' member to AutoRepairData to store the exact tag this
    data was derived from; this allows for easier removal of the tag when
    its associated state is superseded. It also stores the tag that will
    be written to the cluster when a new AutoRepairData record is created.

  - change the default sort order of AutoRepairResult to reflect the
    precedence used when reading tags from an object (a failure, if
    present, is always the resulting state; a success overrides an
    ENOPERM).

  - (trivial) make AutoRepairStatus and AutoRepairData derive Eq and Show.

Signed-off-by: Dato Simó <dato@google.com>
Reviewed-by: Iustin Pop <iustin@google.com>