Code review comment for lp:~jameinel/bzr/2.1-static-tuple-chk-map

This is an update to my earlier chk_map work (which Andrew had approved.)

The main changes are

1) I have benchmarked it as being specifically helpful. Namely it saves 30MB during 'bzr branch launchpad'. (548MB w/ bzr.dev => 518MB)

2) I changed it temporarily to require StaticTuples everywhere, and then updated the code in 'inventory.py' and 'groupcompress_repo.py' that were touching things directly.

3) I then updated the interface so that LeafNode and InternalNode and things underneath them require exactly StaticTuples, but CHKMap itself will cast to StaticTuple on demand. This seemed a fair way to migrate. Production code should never be poking at the internals of the nodes, and instead work via the CHKMap api.

This should help avoid spurious failures in the short term, and still encourage good habits in the long term.