Allow #[error(transparent)] on fields

The crate I'm currently working on has code like the following:

```rust
#[derive(Debug, Clone, Error)]
pub struct Error {
pub(crate) kind: ErrorKind,
backtrack: Option,
}

#[derive(Debug, Error)]
pub enum ErrorKind {
// error variants go here
}
```

If I understand correctly, this doesn't currently allow the `Error` impl for `Error` to return types from the `ErrorKind` variants, which seems like it would be useful. I thought I might try `#[error(transparent)]` on `Error`'s `kind` field, but that's not allowed. Is there currently a way to support this scenario? From the description of `error(transparent)` it seems like it would be a good fit for this, but maybe there's a good reason not to allow this.