Enforce braces around iterator bodies

[lukaszcz]

Currently, the braces are optional. I think we should make them mandatory to make the syntax more consistent.

For example:

for (acc := t) (x in d) {
    deleteWith p x acc
}

Now we also allow:

for (acc := t) (x in d)
  deleteWith p x acc

[lukaszcz]

The formatting is also sometimes weird, e.g.,

  for (acc := empty) (x in tree1)
    {if
      | isMember x tree2 := insert x acc
      | else := acc};

It should be:

  for (acc := empty) (x in tree1) {
    if
     | isMember x tree2 := insert x acc
     | else := acc
  };

[lukaszcz]

Actually, I propose to have a separate case for single-line iterator applications without braces but enforcing the parentheses around the body:

map (x in s) (key x)

This would be formatted differently than the version with braces where newlines would be enforced:

map (x in s) {
  key x
}