[Rust] Implement AsRef<[u8]> on flatbuffers::Array<'a, u8, N>

[dsxmachina]

Hello, this is a suggestion for more ergonomic handling of flatbuffer types.

When I have a byte array in a flatbuffer type, e.g. like this:

struct SignatureWithKey {
  key_bytes: [ubyte: 32];
  sig_bytes: [ubyte: 64];
}

The resulting type is a flatbuffers::Array, that internally just wraps a &[u8].

It would be nice, if these arrays (if they are byte arrays) would automatically implement the AsRef<[u8]> trait from the standard library, to easily integrate in existing interfaces of other crates.

Right now, if you want to use the bytes in some function (e.g. a hash function from another crate), you always have to re-build your slice back from the pointer, which requires an unsafe operation and a copy.