Along with this, support for an optional argument to the "num_children"

method of a Python synthetic child provider has also been added. These have

been added with the following use case in mind:

Synthetic child providers currently have a method "has_children" and

"num_children". While the former is good enough to know if there are

children, it does not give any insight into how many children there are.

Though the latter serves this purpose, calculating the number for children

of a data structure could be an O(N) operation if the data structure has N

children. The new method added in this change provide a middle ground.

One can call GetNumChildren(K) to know if a child exists at an index K

which can be as large as the callers tolerance can be. If the caller wants

to know about children beyond K, it can make an other call with 2K. If the

synthetic child provider maintains state about it counting till K

previosly, then the next call is only an O(K) operation. Infact, all

calls made progressively with steps of K will be O(K) operations.