Make iDestruct more powerful for destructing ghost ownership.

It is now able to destruct:
- [own γ (a1 ⋅ a1)] into [own γ a1] and [own γ a2]
- [own γ a] into [own γ a] and [own γ a] if [a] is persistent
- [own γ (a,b)] by proceeding recursively.
- [own γ (Some a)] by preceeding resursively.