sim: clean up child handling

sim: clean up child handling
The old code for handling SimObject children was kind of messy,
with children stored both in _values and _children, and
inconsistent and potentially buggy handling of SimObject
vectors.  Now children are always stored in _children, and
SimObject vectors are consistently handled using the
SimObjectVector class.

Also, by deferring the parenting of SimObject-valued parameters
until the end (instead of doing it at assignment), we eliminate
the hole where one could assign a vector of SimObjects to a
parameter then append to that vector, with the appended objects
never getting parented properly.

This patch induces small stats changes in tests with data races
due to changes in the object creation & initialization order.
The new code does object vectors in order and so should be more
stable.