autoformatting: if-then-else chaining increases indentation about smlfmt

[shwestrick]

Some progress on this in 6c468d5

Added a combinator called besideAndAbove which is like beside, except it causes the second doc to use the indentation of the first.

    d1       d2           besideAndAbove (d1, d2)
  .......   ****             .......
  .......   ***    ====>     .......
  .....                      .....****
                             ***

Using besideAndAbove to attach the else-branch of a conditional, we get formatting like this:

val x =
  if b1 then
    e1
  else if b2 then
    e2
  else if b3 then
    e3
  else if b4 then
    e4
  else if b5 then
    e5
  else if b6 then e6 else if b7 then e7 else e8

This feels like a step in the right direction. To fix the weird final line of the above example, it seems like we would need to traverse the children of an if/then/else to find the whole chain, before formatting.

[shwestrick]

Although I suppose the irony here is that, if we traverse the children in the CST to identify the whole chain, then it's possible to implement the desired formatting in terms of the old combinators 🥲

[shwestrick]

Fixed by #34 :)