Bug 1437, Bug 1438 NormalizedNode Stream Writer API and Implementation

Initial draft of NormalizedNode Stream Writer API, along
with initial implementation which produces immutable NormalizedNodes.

Change-Id: I45513d88eefca7aa070e6679ccfdedf358266d59
Signed-off-by: Tony Tkacik <ttkacik@cisco.com>
Merge "Bug 1437, Bug 1438 NormalizedNode Stream Writer API and Implementation"