Conditional OpenOp / CloseOp

In `ktfmt`, I'd like to format the following:

```
fun longName() = coroutineScope {
foo()
//
}
```

but if it doesn't fit in a single line, I'd like to get

```
fun longName() =
coroutineScope {
foo()
//
}
```

I only managed to create the first form by _not_ wrapping the lambda in an `OpenOp` / `CloseOp` block, while the second form arises naturally from such a block around `coroutine { … }`.

One way to solve this is to create a conditional `OpenOp` / `CloseOp`, similarly to `Indent.If`, and then emit a block that materializes only if there had been a break after the `=`.

What do you think?

(I admit this isn't super clear, let me know if I should make another attempt at explaining this)

cc @cushon @bethcutler