Handle loops

[mailund]

The reason that I cannot handle loops right now is that I wrap the function body in a repeat loop and use next to replace recursive calls. If I allow loops, and permit recursive function calls inside them, then I would need to be able to jump to the beginning of the outermost loop, whereas the call to next only gets me out of the inner-most loop.

A first step towards handling loops would be to allow them as long as we don't call recursively inside them. A more challenging step is handling the general case... I don't know how to do that yet.

[mailund]

The new non-local return solution to handling with expressions will also make it possible to handle loops.