Implement calling of Futhark Server programs from the interpreter

[vox9]

I added ugly starter code. Please don't roast me too hard, I prefer my meat medium rare.

[vox9]

At the time being, most utility functions of the interpreter are unsupported. This will change, when I have the time to do so. Furthermore, primitive values stored anywhere inside arrays are lazy pulled from the server. Right now, you have to force that by performing a binary operation on them in the end. This should be an easy fix, but not tonight :)

[athas]

This deserves a closer look than I have time for today. However, I am somewhat suspicious of the three modules added to Futhark.Util, as I do not envision they will actually ever be used outside the interpreter. I don't think anything is gained by making them look more general than they really are, and I would strongly prefer moving them under the Language.Futhark.Interpreter hierarchy, to emphasize that this is their home (I also have a long-term dream of separating the interpreter further from the compiler, but let's not worry too much about that).

[athas]

Also, run the formatter and kill the warnings. I won't do that here, because that would probably result in ruinous conflicts with your own upstream code.

[athas]

Actually I am going to reformat this and fix all the warnings, because there are other things I also want to modify. Make sure to pull after that.

[athas]

Reformat done. I also fixed some warnings while I was at it.

[athas]

I have beefed up the futhark eval user experience significantly. Note that it now only uses compiled code when --backend is passed.

[vox9]

Looking good, but remember that only the C server currently supports all the necessary commands.

[athas]

That is the only one that matters.