Better representation of enums

[vlisivka]

I need to parse log of protobuf messages, generated by {:?}, back into protobuf objects, to replay messages. I forked RON deserializer, and made few changes to be able to deserialize {:?} representation. However, I cannot deserialize prost objects, because they use i32 instead of enum to represent enum, so information about actual type of field is lost. Unfortunately, this is limitation of Rust language itself. I hope, it will be fixed soon when RFC 2008 #[non_exhaustive] will be implemented and will work properly for #[repr(C)] enum's.

As workaround, I propose to use union(enum, i32) instead of plain i32, to save information about type. My proof of concept can be found here:
https://play.rust-lang.org/?gist=6131d0fcadd0bf2bba48a3efe26278f0

prost already generates is_valid() method for enum's, so it will not be hard to add generic union, use it instead of i32, and implement IsValidEnum trait for every protobuf enum.

[vlisivka]

Closed in favor of #43, because #[non_exhaustive] tag is stabilized/