nodemaker.create_new_mutable_directory: pack_children() in initial_contents=

nodemaker.create_new_mutable_directory: pack_children() in initial_contents=
instead of creating an empty file and then adding the children later.

This should speed up mkdir(initial_children) considerably, removing two
roundtrips and an entire read-modify-write cycle, probably bringing it down
to a single roundtrip. A quick test (against the volunteergrid) suggests a
30% speedup.

test_dirnode: add new tests to enforce the restrictions that interfaces.py
claims for create_new_mutable_directory(): no UnknownNodes, metadata dicts