It is no longer possible to declare values in the interpreter

[anfelor]

When trying to declare a value in the Koka interpreter 3.1.1, we get the following error:

> val a = 1
src/Interpreter/Interpret.hs:(119,8)-(244,36): Non-exhaustive patterns in case

It used to be possible to declare values in the interpreter, until the code implementing this was commented out in f9f4fc0. It is not too bad that this functionality is gone, since we can use the vscode interface instead, but I would nonetheless prefer it if this still worked.