Add single line if block support for single expression

[Sjael]

What did you expect?

if false { continue }
let Some(x) = opt else { return };

What did you get?

if false { 
    continue  
}
let Some(x) = opt else { return };

cargo fmt puts let-else on a single line, and it could do the same to if blocks that have a single expression (i.e. return, break, or continue). That way there can be a consistent style for all guards.

[lukasjuhrich]

I hope piggybacking is okay, but I expected the same for if let: the line if let Some(r) = right { stack.push(*r) }, reaching column 53, will be broken even with --config use_small_heuristics=max.