shared/json: optimize appending objects to arrays

shared/json: optimize appending objects to arrays

When repeatedly appending an object to a growing array, we would create a new
array larger by one slot, insert all the old entries and the new element with
ref count bumps into the new array, and then unref the old array.

This would cause problems when building an array with more than a few thousand
elements. If userdbctl is modified to construct an array,
'userdbctl --json=pretty group >/dev/null' with 31k groups:
0.74s   (existing code)
102.17s (returning an array)
0.79s   (with this patch)

We append arrays in various places, so it seems nice to make this generally
fast.