Add vtable info

[zhuyutian57]

Though TraitDef and ImplDef are provided, it seems there is no way to get vtable information.

In MIR, a virtual function call can be invoked by InstanceKind::Virtual { idx }, where idx is the index in a vtable_entries. Is there any way to retrieve infomation such as vtable_entries in MIR?

Moreover, we can get the trait items from TraitDef.associated_items(). Is there any relationship between InstanceKind::Virtual { idx } and the associated_item().