[MERGE] Implement xml8 serializer

[Aaron Bentley]

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Hi all,

This patch implements the xml8 serializer which will be used by the
upcoming pack-1.4 format.

Format 8 is based on format 6, the serializer used for rich-root and
rich-root-pack.  Its only physical difference is in the format numbers.
 Format 6's revisions (but not inventories) bore the format 5 marker, so
it is impossible to tell what their inventory_sha1 refers to without
other information.  Format 8 will guarantee that we know what inventory
format corresponds with a given revision.

pack-1.4 will validate inventory sha1s.  When converting into it, if an
inventory sha1 fails to convert, I plan to compare against the following
historical inventory formats: format 5, format 5 with revision_id,
format 6 and format 7.

This patch changes the inheritance so that older formats inherit from
new ones.  This means that the current format is all in one place, and
we can drop support for old formats easily.  It also makes it easy for
future formats to use the same numbers for their revision and inventory
formats.

Aaron
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