Fix for rust commit 3dcd2157403163789aaf21a9ab3c4d30a7c6494d 'Switch to ...

...purely namespaced enums'
Remove the `Decode` and `Encode` prefixes on the variant names, but don’t re-export them. Take advantage on the new language feature! @lifthrasiir, what do you think? By the way, the language change is https://github.com/rust-lang/rust/pull/18973