Add some nullable annotations (#700)

* Add some nullable annotations

* Remove /*!*/

* Cleanup

* Fix tests and cleanup

Author: slozier

Src/IronPython/IronPython.csproj

@@ -55,6 +55,10 @@
       <PrivateAssets>all</PrivateAssets>
       <IncludeAssets>runtime; build; native; contentfiles; analyzers</IncludeAssets>
     </PackageReference>
+    <PackageReference Include="Nullable" Version="1.1.1">
+      <PrivateAssets>all</PrivateAssets>
+      <IncludeAssets>runtime; build; native; contentfiles; analyzers; buildtransitive</IncludeAssets>
+    </PackageReference>
   </ItemGroup>
 
   <ItemGroup Condition=" '$(TargetFramework)' == 'netcoreapp2.1' or '$(TargetFramework)' == 'netstandard2.0' ">