Remove returned pointers to primitive types from the generated C#

[ddobrev]

This can be achieved using the new ref returns from C# 7. It's easy to generate the code but what's more problematic is the automatic compilation of the generated code. There are 3 options there:

1. Force users to upgrade to VS 2017 (requires Support for VS 2017 #785 to be fixed first);
2. Have users install just https://www.nuget.org/packages/Microsoft.Net.Compilers/ replacing their compilers;
3. Ship the entire compiler along with C++#.

All of these have serious flaws so a decision must be taken in advance.

[tritao]

Good idea Dimitar, as an alternative to the options you provided we could also keep the old code path and use it if the user is targeting < VS 2017.