#[derive(ULE, VarULE)]` could be more useful for simple wrapper types

[Manishearth]

In general we nudge people towards #[make_ule] and #[make_varule], but there is a small use case for the direct derives: when you're making a simple wrapper type around an existing ULE or VarULE type.

It would be nice if #[derive(ULE)] could be asked to generate AsULE and ZeroMapKV implementations, and #[derive(VarULE)] could be asked to generate Serialize, Ord (etc), and ZeroMapKV