Rust should respect "private" annotations

[Casper]

The generated code for Java and Swift restrict visibility of flatbuffers types if they're annotated with (private). The semantics are "package private" Java, which I think maps on to pub(crate) in Rust. It should be easy enough to implement if and when any users ask for it

[mustiikhalil]

I can pick this up since I'm trying to get more into rust.