format: retain comments in remaining types

[avh4]

• retain comments in type parentheses
  • after opening paren
  • before closing paren
• retain comments in function types
  • before arrow
  • after arrow
• retain comments before each type argument
• retain comments in union type declarations
  • between type keyword and name
  • before each argument
  • between last argument (or name) and =
  • between = or | and each variant
  • before each argument to a variant
  • between the end of a variant and the next |
  • after the last variant (indented)