Rework enums in rust. (#6098)

* Rework enums in rust.

They're now a unit struct, rather than an enum. This is a
backwards incompatible change but the previous version had UB
and was also backwards incompatible so...

* Update and test sample rust flatbuffers

* Use bitflags crate to properly support rust enums.

Previously, the bitflags attribute was just ignored. This is a breaking change
as the bitflgs API is not like a normal rust enum (duh).

* variant_name() -> Option<_>

* repr transparent

* Reexport bitflags from flatbuffers

* Make bitflags constants CamelCase, matching normal enums

* Deprecate c-style associated enum constants

Co-authored-by: Casper Neo <cneo@google.com>

rust/flatbuffers/Cargo.toml

@@ -12,3 +12,4 @@ categories = ["encoding", "data-structures", "memory-management"]
 
 [dependencies]
 smallvec = "1.0"
+bitflags = "1.2"

rust/flatbuffers/src/lib.rs

@@ -38,6 +38,7 @@ mod vector;
 mod vtable;
 mod vtable_writer;
 
+pub use bitflags;
 pub use crate::builder::FlatBufferBuilder;
 pub use crate::endian_scalar::{
     byte_swap_f32, byte_swap_f64, emplace_scalar, read_scalar, read_scalar_at, EndianScalar,