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 #8550
• fixes [Rust] SafeTable is not publicly visible in reflection library #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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[jtdavis777]

hey @Muon - thank you for the PR! it looks like you may have some merge conflicts that need resolved. This also has quite a bit in it and I don't think we have an active Rust maintainer at the moment. It may help out a lot to either rebase this PR to have each commit address a separate issue, or break this into multiple smaller PRs that are easier to digest. I'm happy to help out how I can but I'm a total noob in Rust.