* 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>
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