Improve Rust Flatbuffers crate

[Muon]

This PR fixes several bugs and adds support for missing features to the flatbuffers and flatbuffers_reflection crates.

• fixes [Rust] [Reflection] get_field_vector() only works for vectors of primitive types google/flatbuffers#8550
• fixes [Rust] SafeTable is not publicly visible in reflection library google/flatbuffers#8548
• adds reflection support for vectors of values of unknown type
• adds verification support for vectors of unions
• makes the unsafe get_field_* functions infallible as their success is implied by the unsafe precondition
• adds the unsafe function Vector::cast() for casting between vector types
• exposes get_type_size()
• adds Struct::bytes() to get a slice to a byte buffer which starts with the struct value

[bennetthardwick]

I don't know much about flatbuffers but I'm sure this will get covered enough by usage in the message path changes.