Fix evaluation of nested classes in the csharp REPL.

This PR is a fix for the csharp REPL as it throws a NULL exception when trying to evaluate nested classes e.g.
```c#
class A { class B { } }
```
This is a fix for https://bugzilla.xamarin.com/show_bug.cgi?id=19876

I was unable to create a test for this as I found no charp specific tests in the project. If this is needed please advice on how to proceed.

To be able to compile the csharp project I needed also to fix a reference to cs-parser.cs in the Mono.Csharp.csproj as it wouldn't build otherwise. If already fixed I can pull that commit out of this PR.

Note:
* Note, the mcs compiler does not have this issue. This only happens only in the csharp REPL.
* When I run <code>make run-test</code> on clean latest build I get 5 error, but <code>make compiler-tests</code> works before and after the fix.