improve comment/node association

[josharian]

We could start by stealing some of the Go heuristics, which definitely have some shortcomings but work well enough for common cases in practice:

A CommentGroup represents a sequence of comments with no other tokens and no empty lines between.

A comment group g is associated with a node n if:

• g starts on the same line as n ends
• g starts on the line immediately following n, and there is at least one empty line after g and before the next node
• g starts before n and is not associated to the node before n via the previous rules

[It] tries to associate a comment group to the "largest" node possible: For instance, if the comment is a line comment trailing an assignment, the comment is associated with the entire assignment rather than just the last operand in the assignment.

[pokey]

Use cases:

• "item" scope could include the comment associated with the item
• Other scopes could include their associated comments