format: retain comments in case and if expressions

[avh4]

• retain comments in case expressions:
  • between case keyword and condition
  • between condition and of keyword
  • between of keyword and first branch
  • for each branch:
    • between pattern and ->
    • between -> and expression
    • after the expression (indented)
  • between branches
• retain comments in if expressions:
  • for each "if/then" clause:
    • between if keyword and condition
    • between condition and then keyword
    • between then keyword and expression
    • between expression and else keyword
  • between else keyword and if keyword (will be moved to after if)
  • between else keyword and final else expression
  • after final else expression (indented)