Serde Deserialize errors do not set the field index

A field that returns a serde deserialize error will always have the `field` set to `None`.

```rust
impl SerdeError for DeserializeError {
fn custom(msg: T) -> DeserializeError {
DeserializeError { field: None, kind: DEK::Message(msg.to_string()) }
}
}
```

It does not appear we can determine the field that failed to deserialize from serde to report to the user.

One option is to modify the `deserialize_string_record` function to manually set the `err.field` in the `map_err` closure

```rust
D::deserialize(&mut deser).map_err(|mut err| {
err.set_field(Some(deser.0.field.saturating_sub(1));
Error::new(ErrorKind::Deserialize {
pos: record.position().map(Clone::clone),
err,
})
})
```

I am unsure if this is a valid solution or if there are edge cases that will cause the field index to not be accurate.