[MERGE] Inventory serialisation tweaks

[Robert Collins]

This shaves about 20% off inventory serialisation in the serialiser, and
also tweaks the api to fit better with the knit api for storage of the
inventory.

All tests pass, no xml canonical form changes are made at all. And there
are some drive by cleanups of the xml4 disabled writer.

NEWS has more details.

-Rob
-- 
GPG key available at: <http://www.robertcollins.net/keys.txt>.
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