[Ur] sanity checking

[Adam Chlipala]

Marc Weber wrote:
> It doesn't matter whether this is a gcc bug or not.
> It would be nice if we could trust the result even if gcc contained a
> bug. :-/  I've no clue how often this code resulting in bad behaviour is
> generated by urweb.
>
> That's one reason why a js target could be nice. because you don't have
> to know C extensions ..
>    

To me, it just seems futile to try to build a compiler based on 
intermediate C code, without being sure that the underlying C compiler 
works correctly.  The Ur/Web code you wrote was non-idiomatic, which led 
to especially weird C code, so I'm not too worried about this issue for 
the future.