[WIP] Implement "Add Open" code fix (#2067)

* port lexer symbol lookup logic from VFPT and use it in DocumentHighlightsService

* QuickInfoProvider uses lexer symbol

* remove tryClassifyAtPosition

* do not show quick info if FCS return a list of single FSharpTooltipElement.None

* use standard Option combinators

remove Pervasive.fs

* AddOpenCodeFixProvider WIP

* wip

* do not use new functions from Option module introduced in F# 4.1

* it works

* cosmetics

* filter out duplicated open namespace code fixes

src/fsharp/FSharp.LanguageService.Compiler/FSharp.LanguageService.Compiler.fsproj

@@ -539,6 +539,9 @@
     <Compile Include="..\vs\ServiceStructure.fs">
       <Link>Service/ServiceStructure.fs</Link>
     </Compile>
+    <Compile Include="..\vs\ServiceAssemblyContent.fs">
+      <Link>Service/ServiceAssemblyContent.fs</Link>
+    </Compile>
     <Compile Include="..\vs\service.fsi">
       <Link>Service/service.fsi</Link>
     </Compile>