chains: prefer to use the next line for an expression, if using the same line would...

chains: prefer to use the next line for an expression, if using the same line would introduce an open block or similar

This problem came to light due to the chains changes, but effects other code too. It only happens rarely, e.g.,

before this fix:
```
    match foo {
        MacroArgKind::Delimited(ref delim_tok, ref args) => rewrite_delimited_inner(
            delim_tok,
            args,
        ).map(|(lhs, inner, rhs)| format!("{}{}{}", lhs, inner, rhs)),
    };

```

after:
```
    match foo {
        MacroArgKind::Delimited(ref delim_tok, ref args) => {
            rewrite_delimited_inner(delim_tok, args)
                .map(|(lhs, inner, rhs)| format!("{}{}{}", lhs, inner, rhs))
        }
    }

```