lib: add incremental json vty output

Add some new lib apis that allow us to incrementally output
a json object hierarchy. Many leaf objects can be freed
as soon as they've been output. Parent container objects
can be kept "open" during long-running iterations, and then
"closed" once their children have been processed. This
incremental approach reduces the number of json objects
in memory; only "open" containers and a limited number of
leaf objects need to be in memory.

Signed-off-by: Mark Stapp <mjs@cisco.com>

Author: Mark Stapp