Combine .NET Framework and .NET Core fsproj and sln files

[chinwobble]

With VS 15.6, multi-targeting projects appear to be working properly in the IDE.

I suggest that we combine the project files if possible.

I ran into errors when trying to benchmark some tests on .NET Core where unrelated fsproj for .NET framework was throwing errors despite not even being used. After deleting the .NET framework fsproj, the tests ran properly.
The value "" of the "Project" attribute in element <Import> is invalid. Parameter "path" cannot have zero length.
I am hoping that combining will make a lot of things easier like benchmarking under different build targets.

[baronfel]

This is done now! #325 unified the project and solution files.