Extract out AST-collecting-walker to a separate function + abstract c…

…lass (ionide#1154)

* extract out AST-collecting-walker to a separate function + abstract class and use it for range aggregation

* better factoring of the syntax-walking code

src/FsAutoComplete.Core/Commands.fs

@@ -206,7 +206,7 @@ module Commands =
   let getRangesAtPosition (getParseResultsForFile: _ -> Async<Result<FSharpParseFileResults, _>>) file positions =
     asyncResult {
       let! ast = getParseResultsForFile file
-      return positions |> List.map (UntypedAstUtils.getRangesAtPosition ast.ParseTree)
+      return positions |> List.map (FoldingRange.getRangesAtPosition ast.ParseTree)
     }
 
   let scopesForFile