Store top fully encompassing ddvt of a particle.

Optimizes lookup of ddvts to update for a particle.
Embed world data in particle.
fixup! Store top fully encompassing ddvt of a particle.
fixup! Optimize _phDdvtNextChild, _phDdvtPairNext
fixup! Optimize phAppend, phRemove
Add phShift.
Use phShift in world collision list work.
Deref world ddvtpair leaf iterators and particle a.