[merge] Improved XML Writer (5-10x faster)

[John Arbash Meinel]

Well, I looked into Robert's work on updating the XML serializer to have
a custom writer, rather than going through ElementTree's slow writer.
And it turns out, we can take a 2s operation (to write out a 10K tree),
and make it all the way down to a 250-400ms operation.

I was even able to preserve the ElementTree property that writing the
XML uses numeric entity references, rather than using real UTF-8 encoding.

This means that the change is fully compatible with old versions of bzr.

Because of the --cache-dir flag, you can also substitute a real kernel
tree into the benchmarks. So I've done that.

These are my results:

2044ms	kernel_like_tree write_inventory()
 365ms	kernel_like_tree custom write_inventory()
 281ms	kernel_like_tree custom cached write_inventory()

5029ms	real_kernel_tree write_inventory()
1050ms 	real_kernel_tree custom write_inventory()
 786ms	real_kernel_tree custom cached write_inventory()

The 'cached' write is writing out the same inventory a second time. With
the Unicode => encoded_entity mapping already saved. I don't know if we
want to clear out the cache at the end of writing the inventory, I've
commented out the function that does it, so that I can do my tests.

The only time it is really useful is when we write an inventory to the
repository, and then turn around and write the basis inventory. Since a
lot of the file_id and path stuff will be cached.

I think it would be nice to see this merged for 0.10.

John
=:->
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