Iterator support for serde deserialization

I have a file that is a sequence of serialized structs and I want to read them, one by one. I believe this is quite common situation. This is currently a bit hard, I have the following code:

```rust
while let Ok(record) = from_read::<_, Record>(&mut reader) {
// Do stuff with record here
}
// Check that the above loop ended (errored) on eof, not something else
let mut ctl_buf = [0];
assert_eq!(0, reader.read(&mut ctl_buf).unwrap());
```

This checks first tries to repeatedly read a value from the reader until it fails and then checks that it reached the end of the file, not some other error. This, however, might also mean an EOF *in the middle* of the record, not only on the boundary (eg. if there's no more record).

There could be two ways to allow handling the situation in a better way:
* Have an error type that can distinguish the situations (eg. an EOF before we read any data, an EOF in the middle of the record…).
* Have an iterator reader, similar to `serde-json` has in [`StreamDeserializer`](https://docs.rs/serde_json/0.9.9/serde_json/de/struct.StreamDeserializer.html).

Would it be hard to add either of these?