add simple FSharpKernel wrapper around fsi.exe

[brettfo]

This is more of a proof-of-concept until we decide how we want to properly refactor fsi.

Since we're forced to scrape stdin/stdout, all returned values are forced to text/plain (hence no formatter support because everything is System.String); this will change once we have a proper kernel.

Includes moving most of CSharpKernelTestBase.cs into a common KernelTestBase.cs.

[jonsequitur]

I have some changes in flight here as well and I'm leaning toward always using the same default set of extensions here as we use in the app at runtime. There's really only one supported configuration at the moment so we should test that.

[brettfo]

Currently I think the .UseDefaultRendering() extension should stay only in CSharpKernelTestBase.cs because the F# kernel doesn't support any of that yet since it's really just a thin wrapper/scraper around fsi.

The other alternative is to broaden the extension methods to switch on specific types like so:

public static KernelBase UseDefaultRendering(this KernelBase kernel)
{
    switch (kernel)
    {
        case CSharpKernel csharpKernel:
            csharpKernel.UseDefaultRendering();
            break;
        case FSharpKernel fsharpKernel:
            // noop
            break;
    }

    return kernel;
}

public static KernelBase UseDefaultRendering(this CSharpKernel kernel)
{
    // do C#-specific stuff
}

Thoughts?

[jonsequitur]

Rather than change the type that UseDefaultRendering accepts, maybe we should add an overload for FSharpKernel. The implication of the proposed method is that it should work for all kernel types, which isn't really possible since we can't know all of the possible implementations of KernelBase.

[brettfo]

I added void SetDefaultRendering() to IKernel so C# can implement it and F#'s is currently a noop.

Also updated to reflect your recent changes to master.

[jonsequitur]

I don't think that belongs on IKernel. Rendering is a fairly separate concern.

[brettfo]

[jonsequitur]

There's no generalization of default rendering that makes sense across all possible kernels, and the concept may not be something we even want to commit to for the two existing kernels. Let's keep the implementations specific to the cases where we understand what this means and keep the interface simpler for now.

[brettfo]

Then move this back to be specific to the C# kernel? That's where we originally started which seems at odds with how I interpreted this comment, specifically, I read this:

maybe we should add an overload for FSharpKernel.

As

move this method to KernelBase

But to properly support CompositeKernel, the method needed to be on IKernel, instead.

[jonsequitur]

Sorry. I meant that we should have two methods, one for CSharpKernel and one for FSharpKernel. Their implementations are different and I don't know if this method will make sense for all kernels.

[brettfo]

Added commit 03e8866, let me know what you think.

[brettfo]

Rebased to handle recent changes in master.

[brettfo]

Ping @jonsequitur for review.

[colombod]

Looks good to me