BUG-1668: make sure MapEntryNodes emit key children first

As per RFC6020, the XML representation requires the key entries to be
emitted first. It takes some doing, but this patch does exactly that.

Change-Id: If1b8155f914475224389609cc43e244e2e431f23
Signed-off-by: Robert Varga <rovarga@cisco.com>
NormalizedNodeWriter: emit sizing hints

This patch adds the minimum smarts to allow emitting child sizing hints.

Change-Id: I79a4728db1ec1cb9460e5750d3cce099185add94
Signed-off-by: Robert Varga <rovarga@cisco.com>