Formatter: add braces when the iterator body is not enclosed

[lukaszcz]

• Closes Enforce braces around iterator bodies #3091
• Formatter adds braces when the body is not enclosed in braces or parentheses. Braces-enclosed body is always printed as a block on a new line:

for (acc := 0) (x in lst) {
  x + acc
}

• If the body is enclosed in ordinary parentheses, then they are preserved and the iterator is printed on a single line, if possible:

for (acc := 0) (x in lst) (x + acc)

This is sometimes useful when you want iterator application as an argument to something.

[janmasrovira]

It might be confusing to support both parentheses and braces with the same meaning. Should we always use parentheses?

[lukaszcz]

Yeah, maybe we should discuss this.

My idea was that the formatter behaves differently with braces v.s. parentheses. So you can use parentheses if you want to prevent the line break, but normally you would use braces. There are a few cases I noticed in the code where it actually makes sense to have iterator application as an argument, and then you don't want a line break. Other options are to use only braces and either try to detect when to do line breaks or always do line breaks (which results in poor formatting in the rare cases when you want iterator application as an argument).

Imho braces look better when something spans multiple lines -- they seem to indicate you have a "block" of something. We use them for this kind of grouping with e.g. case expressions.

[lukaszcz]

The way I thought about it: you either have an atom or a block enclosed in braces. Since an atom can be an expression enclosed in parentheses, that's where they come from.

For now, having an expression with multiple atoms for a body is still allowed, but the formatter will put braces around it and we should disallow it in the future.