Multiline-friendly toString()

One of the things we did in Truth a while back was to print values differently if they contain newlines. So, for example, you would see:

```
expected: foo
but was : bar
````

But you'd see:

```
expected:
public class Foo {
Foo() {}
}
but was:
public class Bar {
Bar() {}
}
````

I would speculate (but it's just speculation) that this might be a nice feature for AutoValue `toString()` implementations, too.

(Even in the case in which fields _aren't_ multiline, a multiline `toString()` can be nice in some cases: I think Truth has gotten reports that AutoValue `toString()` (like, to be fair, almost all `toString()` implementations) makes it hard to see which field differs when there are a lot of fields. But of course one-line `toString()` is nice in plenty of cases, too, so I wouldn't advocate for _always_ going multiline (nor, probably, for making it configurable). It's just a nice additional advantage in the cases in which multiline is already justified.)

(It's also possible that Truth should have more special handling of AutoValue types in some cases.)