Hiffy-related bits assume enum discriminants are signed, sometimes bytes

[cbiffle]

This is a follow-on for #468. In #469 I fixed the cases where enums were being read from target memory (or memory images), but that's not every case where we do enum parsing.

There are several cases in Hiffy-related commands like rpc where we "parse" enums from data returned in an array of u32s. I didn't go ahead and fix these, because there's some cross-system shenanigans happening there that obscure whether the data is being e.g. zero-extended. Instead, I slapped TODOs on them.

At the time of this writing the affected crates were

• cmd/rpc
• cmd/validate
• humility-idol
• humility-hiffy