Vtable refactor: use `vtable_entries`, sanity check completeness

[avanhatt]

Ideally, our implementation of vtables for dyn trait matching would not rely so heavily on pretty name string matching, and instead more closely match the rustc implementation. Creating this issue to track the overall refactor.

[avanhatt]

This might not be necessary now that we look up dynamic calls using the full symbol name. The only name string matching left is for the trait and function name stored in the table itself.