format: retain top-level comments

[avh4]

• retain comments between top-level declarations
  • ... (general case)
  • ... after custom type declarations
  • ... after value declarations whose body is a case expression
• retain comments between import statements and the first top-level declaration

As a side note, to capture the comments after a sequence of items that are parsed with a recursive function, the comments between elements were temporarily captured. In those cases, I went ahead and attached those "between" comments to the AST, even though they aren't output yet. This happened for import statements, case branches, type arguments, and custom type variants.

[robinheghan]

Would you mind rebasing this?

[avh4]

Updated!

[robinheghan]

Great work!