Update vtable implementation for upcoming Rust trait upcasting

[avanhatt]

Rust has an in-progress proposal to add a trait upcasting coercion feature for dynamic trait objects. This
will allow casting between dynamic trait object types as long as the source type is a subtrait of the destination type.

We'll need to update our implementation as soon as all components of this issue land in Rust. In particular, the construction of vtables and the casting in rvalue.rs will need to handle indirect access through vtable segments.
rust-lang/rust#65991

[avanhatt]

Handling initial support in #345.