Improve Rust flatbuffers crate

[Muon]

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

• fixes https://github.com/google/flatbuffers/issues/8550
• fixes https://github.com/google/flatbuffers/issues/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

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