Consider line breaking in `let` and match arms

[nrc]

We should consider line breaks after = in let statements and => in match arms if that gives us better formatting.

So, what is better? That is the fun bit. I would say that if without a break we use >1 lines and with we only use 1 then it is better. Also if the line break means we use many fewer lines (more than 2 fewer maybe?). It would be nice to be able to check if we're causing ugly rightward drift too. So add an option for how many spaces indent is ugly, and if we use the same number of lines or fewer and we change the indent from > ugly to < ugly, then it is better. I imagine this will take some experimentation.

[sanxiyn]

We should consider breaking after = in general, not only in let statements.

[marcusklaas]

Related: #269.

[marcusklaas]

This has been implemented for let statements (I'll have to check for assignment in general). Does this still make sense for match arms? I think we always break multi-line arms now.