* Expose `Ast.fold` and `Ast.tryPick`.
* Expose `SyntaxNode.(|Attributes|)`.
* Ensure a few more syntax node cases get hit.

* Put `Ast.foldWhile` back in.

* Add `Ast.exists`.

* Add `Ast.tryNode`.

* Replace uses of `SyntaxTraversal.Traverse` in `FSharpParseFileResults`
  with the appropriate function from the `Ast` module: `exists`,
  `tryPick`, `tryNode`.

* Before, no signature help was offered in a case like this:

  ```fsharp
  [1..10]
  |> List.fold (fun acc _ -> acc) ‸
  |> List.filter (fun x -> x > 3)
  ```

  The service will now offer help for the `state` parameter when the
  cursor ‸ is in that location.

* `FSharpParseFileResults.TryRangeOfFunctionOrMethodBeingApplied` was
  previously returning the range of the (zero-width)
  `SynExpr.ArbitraryAfterError`. It now returns the range of the `*`
  (`op_Multiply`) instead.

* Use `ParsedInput` as the main AST type.

* Move the `position` parameter rightward.

…into ast-funcs

* Add basic examples for `ParsedInput` module functions.

* Merge the existing `SyntaxVisitorBase` docs into the new file.